Surfaces and Substrates in Painting

Surfaces and Substrates in Painting

A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate. Paints and lacquers are coatings that mostly have dual uses of protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is presumably only for the function of preventing corrosion. Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wetability, corrosion resistance, or wear resistance. In other cases, e.g. semiconductor device fabrication (where the substrate is a wafer), the coating adds a completely new property such as a magnetic response or electrical conductivity and forms an essential part of the finished product.

A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for ‘non-all-over’ coatings is that control is needed as to where the coating is to be applied. A number of these non-all-over coating processes are printing processes.

Many industrial coating processes involve the application of a thin film of functional material to a substrate, such as paper, fabric, film, foil, or sheet stock. If the substrate starts and ends the process wound up in a roll, the process may be termed “roll-to-roll” or “web-based” coating. A roll of substrate, when wound through the coating machine, is typically called a web. Coatings may be applied as liquids, gases or solids.

Reference: Wikipedia

Trade Terms

Alkaline cleaner: A cleaner that saponifies (converts oil, grease, or organic compounds a water-soluble soap) and washes away other types of contaminants.

Alligatoring: A paint surface defect that forms cracks resembling alligator hide. Also known as crocodiling.

Blasting: Short term for abrasive blasting using compressed air or pressurized water.

Block filler: A heavily pigmented coating used to fill the pores and voids of cinder or concrete block.

Calcimine: A finish composed of water, calcium carbonate, and glue or size that was used in older construction. It was used primarily on plaster ceilings but was also used on walls or interior masonry and sometimes tinted for decorative purposes.

Chalking: The decomposition of a paint film into a loose powder on the film surface. Concrete: A mixture of portland cement, aggregates, water, and, at times, special additives.

Consolidant: A liquid epoxy consisting of a resin and hardener that, when mixed properly, can be used to repair rotted, dried out, or spongy wood. When dry, it may be sanded, sawed, drilled, and finished like wood.

Detergent: A synthetic organic cleaning agent that is liquid or water soluble and has wetting and emulsifying properties.

Efflorescence: An encrustation of soluble salts, commonly white, deposited on the surface of coatings, stone, brick, plaster, or mortar. It is caused by the leaching of salts or alkalis from mortar or adjacent concrete as moisture moves through them.

Laitance: A thin, weak, brittle layer of cement on a concrete surface. It is usually caused by an overly wet or overworked mixture.

Neutralized: The process used to eliminate excess alkalinity or acidity in concrete or masonry. It is measured in units of pH, with 7 being the desired neutral value.

Parge coat: A mixture of water, portland cement, and sand that is spread onto masonry or poured concrete walls in a thin layer to provide a seal and a smooth finish.

pH: A measure of the alkalinity or acidity of a liquid or solid material. A pH reading of 7 is neutral; greater than 7 is alkaline (basic); less than 7 is acidic. The further from 7 the reading, the more acidic or alkaline the liquid or material is.

Primer: 1. The first coat in a painting operation. It is designed to promote adhesion of subsequent coats. 2. A coating applied to a substrate to improve the adhesion of wall covering adhesive.

Profile: Contour or roughness of a surface. Also known as surface profile, anchor profile, or anchor pattern.

Substrate: Any material to which paint, coating, or wall covering is applied.

Surface: 1. The portion of the substrate to which paint, coating, or wall covering is applied. 2. The finish obtained after paint, coating, or wall covering has been applied.

Wallcovering: Any type of paper, vinyl, fabric, or specialty material that is pasted onto a wall or ceiling for decoration and/or protection.

Wash coat: A thinned coat of paint or stain applied as a primer prior to the final finish.

White coat: A thin, fine plaster coat, also known as a putty coat, applied over a coarse plaster base to obtain the final plaster finish. Years ago, this coat was sometimes tinted to the desired room color to eliminate painting.

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New Exterior Substrates

The primary substrates used in exterior construction are:

  • Wood
  • Metal
  • Masonry, concrete, and stucco
  • Synthetic

Outside surfaces of these substrates are subjected to a variety of extremes. Temperatures may vary from very hot in the summer to extremely cold in the winter. Wind, rain, hail, sleet, snow, and the sun’s ultraviolet rays also attack the surface and affect the life and appearance of the coating. In urban areas, chemicals produced by vehicles and factories further attack and destroy the coating. Some coatings are subjected directly to very corrosive materials such as in chemical plants.

Close-Up View Of Open-Grain Oak And Close-Grain Pine

Common Types Of Softwood And Their Characteristics

Exterior Wood Substrates

Wood is classified as either hardwood or softwood. There are several different methods of classifying wood. The most common method is by its source. Hardwood comes from trees that shed their leaves each year. Softwood is cut from cone-bearing trees, commonly known as evergreens. In this method of classifying wood, some types of softwood may actually be harder than hardwood. For example, fir, a softwood, is harder and stronger than basswood, a hardwood.

Wood may also be divided into two groups according to cell structure. Open-grain wood (Figure 1) has large cells that show tiny openings or pores in the surface. All softwood is classified as close-grain wood. Refer to Table 1 for types of softwood and their characteristics. Table 2 lists common types of hardwood.

Identifying different kinds of wood can be very difficult because some closely resemble each other. For example, ash and white oak are hard to distinguish from each other, as are some pine and spruce. Not only are they the same color, but the grain pattern and weight are about the same. Only the most experienced workers are able to tell the difference.It is possible to get some clues to identifying wood by checking its color and grain, pressing it with your fingernail to see if it dents easily (softwood) or barely dents (hardwood), or by smelling a newly-sanded section for a characteristic odor. Most exterior wood is used in residential environments and thus is subject only to normal weathering and atmospheric conditions.

Construction wood, including plywood, particle or flake board, composite wood siding, pressure- treated wood, and fire-retardant wood is generally made of porous softwood. This usually means that it must be sealed prior to being coated. If the wood has a high oil content, sealers must be used to keep it from bleeding. Many pine varieties contain a sap or pitch which bleeds through many coatings (particularly water-based emulsion paints). Alkyd-oil sealers are usually used to prevent the bleeding. Pressure-treated wood should be allowed to dry sufficiently before coating. Fire-retardant treated wood must be coated with a fire-retardant coating.

Moisture content is also a problem with new wood surfaces. If the wood is too wet, the moisture will try to escape through the surface. If the surface is waterproof, the moisture will probably lift the coating. New wood that has been subjected to rain should be allowed to dry sufficiently before applying a primer, which is the first coat of a painting operation. A moisture meter should be used to make sure that the moisture content is sufficiently low before applying primer. If new wood has been exposed to the sun’s rays for more than several days, it should be sanded before applying a primer.

In general, all new wood should be primed before painting. Wood that bleeds must be primed with a primer-sealer before painting. When considering coating systems for exterior wood surfaces, you must consider the following environmental factors:

  • Level of sun exposure
  • Extent of heat or cold exposure
  • Amount of humidity
  • Other atmospheric or environmental (e.g., corrosive) conditions

Exterior Metal Substrates

Both ferrous (steel) and non-ferrous (no iron content) metals are used in construction today. If the metal substrate is not known, it may be generally classified using a magnet. A magnet will be attracted to ferrous metals but will have little or no attraction to non-ferrous metals,such as brass. Carefully sanding a hidden portion of a metal to the bare surface may reveal zinc or tin plate on steel or the color and surface characteristics of non-ferrous metals. Nonferrous metals include:

Aluminum – Whitish with a dull and soft surface easily scored by sanding.

Stainless steel – Shiny like steel with a relatively hard surface.

Copper – Characteristic bright copper color.

Brass – Generally bright yellow or gold color.

Bronze – Reddish yellow in color (between brass and copper).

Steel

Steel has three contaminants that must be removed prior to painting. The first one is rust, which is caused by oxidation of the metal. The second is mill scale or slag, which is produced during manufacture. The third is a combination of oil and dirt, which is caused by handling. Contaminants can be removed in a variety of ways, including hand sanding, power tools, chemical cleaning, or abrasive blasting. These techniques will be discussed in detail in other modules.

Once the surface has been cleaned, a primer is selected and the surface is primed. The selection of the primer is dependent on several items, such as the surface corrosion pretreatment required, the top coating to be used, and the environment in which the metal surface exists.

Generally, alkyd-oil paints and latex-emulsion paints are used in mild environments. Where heavy corrosion is a concern, corrosion treatment (using phosphate conversion and/or zinc chromate coatings) is applied before the primer. The primer is followed by top coats (where legally permitted) of epoxy, urethane, silicone, chlorinated rubber, vinyl, or phenolics.

Aluminum

Aluminum is a whitish, soft metal that oxidizes much more slowly than steel. It’s oxidation actually provides a protective coating against further corrosion. These oxides sometimes provide a good surface for paint so little preparation is needed after surface cleaning. Aluminum is cleaned primarily by hand or mechanical means. This can be done using power tools, blasting (compressed air abrasive blasting or pressurized water), or chemical cleaning. New aluminum may need to be etched by chemical means to provide the proper profile (contour or roughness of a surface) for paint. Zinc chromates are excellent primers for aluminum; however, there are many others. Be sure to follow the coating manufacturer’s recommendations.

Galvanized Steel

Galvanized (zinc coated) steel is often used for siding, fencing, and roofing. The zinc coating itself is rust resistant. However, galvanized steel coats better if it is weathered. Washing it with an etching solution helps to “weather” or give tooth to the surface. Special primers are also available.

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Copper, Brass, Bronze, And Stainless Steel

Most other metals, such as copper, brass, bronze, and stainless steel do not need to be coated for protection. However, a clear finish may be applied to a clean surface to provide sheen and abrasion protection.

MASONRY, CONCRETE, AND STUCCO SUBSTRATES

These substrates are found in residential, commercial, and industrial construction. Concrete, which is a mixture of water, portland cement, aggregates, and at times, special additives, is used for foundation walls, ceilings, and floors. It may be in either slab or block form. Brick, stone, stucco, gunnite, and fiber-cement panels are used for walls and veneers. Brick and stone are also used for floors.

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Masonry products all produce alkali and require the use of alkali-resistant coatings. Where concrete or gunite is used for water immersion such as swimming pools, waterproof coatings like chlorinated rubber and epoxies are used where permitted. New masonry, stucco, gunite, or concrete must cure for at least 28 days to eliminate moisture. The surfaces must then be cleaned of anyefflorescence (salts deposited on the surface that is leached from mortar and concrete by water), curing compounds, and form release agents. Concrete, gunite, stucco, fiber-cement panels, or masonry walls should be mechanically or hand cleaned and should not be acid etched. All cracks and voids must be filled.

This can be done using a parge coat (a mixture of water, portland cement, and sand) or a commercial block filler (a heavily pigmented coating).

Close-Up View Of Voids In Concrete Block Substrate

The surface must be rinsed and checked for the proper pH range, which is the measure of acidity or alkalinity of the surface. After the surface is dry, it must be checked for moisture content before being coated. New concrete floors should be blasted or cleaned with hand or power tools. Also, they may be acidly etched to remove laitance (a thin, brittle layer of cement on a concrete surface) and loose surface material and provide the proper profile (Figure 6). A good surface is obtained when water will soak into it. Concrete is usually etched with a muriatic acid solution and then neutralized. After the surface has been rinsed and checked for an acceptable pH range and has thoroughly dried, it must be checked for moisture content before being coated.

New brick should be allowed to age for one year before coating. Brick surfaces can be cleaned with a detergent or by blasting. Acid etching is not recommended for vertical surfaces. Coatings used for brick are (where legally permitted) acrylics, epoxy, urethane, chlorinated rubber, silicone, and phenolics.

Close-Up View Of Etched Concrete Substrate Showing Fine Sandpaper Effect (Profile)

Synthetic Substrates

A painter is sometimes required to work on a synthetic (man-made) surface. It may be made of fiberglass or various types of plastic. Vinyl siding is an example of a synthetic surface. For proper preparation and finishing, the type of plastic should first be identified. Next, reference books or the manufacturer should be consulted to obtain instructions on surface repair, preparation, priming, and painting. Some plastics, such as teflon, do not coat well; others, such as flexible vinyl, require special primers or paint additives. Vinyl siding and plastic (ABS, PVC, etc.) pipe can usually be coated with a high-quality latex primer and paint after the surface is scuff-sanded with sandpaper or de-glossed with a chemical de-glosser.
For new construction, the general contractor or architect should be able to furnish the information necessary to identify the plastic.

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New Interior Substrates

The primary substrates used in interior construction are:

  • Drywall/plaster
  • Wood
  • Metal
  • Masonry, concrete, and stucco
  • Synthetic

Generally, interior substrates are not as troublesome as exterior substrates. They are not subjected to extreme weather and atmospheric conditions and usually experience less chemical interference. When preparing or painting interior substrates, you must take precautions to prevent dust or vapor contamination of adjacent areas. Occupants in the work area or in adjacent areas are often relocated while work is in progress.

Plaster Substrates

Two-coat plaster is becoming less common in new construction. New, wet plaster has a high alkalinity which deteriorates oil-based coatings. Wet plaster should be allowed to cure thoroughly before coating. This may take anywhere from 4 to 5 weeks. Plaster should be checked for an acceptable pH range. It must also be checked for moisture before being coated. If the plaster is dry and has an acceptable pH, water-based emulsion paints should be used or acrylic primer sealers should be applied.

Typical Two-Coat Plaster System

Drywall (Wallboard) Substrates

Today, most interior walls are constructed of drywall. Drywall is a sandwich of gypsum in between coverings of paper. These materials are produced in sheets of various sizes and thicknesses. They are screwed, nailed, or glued to wall studs and finished with paper or fiber- mesh tape and/or joint compound to eliminate joints and fastener heads When painting drywall, some water-based paints may be applied directly to the surface without primer. Oil-based paints should be used with the correct primers and sealers. Under critical lighting conditions or when a high-luster finish or a thin wall covering will be applied drywall is covered with a thin skim (veneer) coat of joint compound and then lightly touched up by sanding to provide a very smooth surface.

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Wood Substrates

Interior wood substrates are generally not exposed to as harsh an environment as exterior wood substrates. However, surface preparation and paint selection may be just as demanding. Tables 1 and 2 described the characteristics of common types of wood. Oak, for example, is an open-grain hardwood. Because of this, oak will not produce a smooth surface unless it is filled. Pine, on the other hand, does not need to be filled.

An exception to this is where a stain and/or a clear finish is used. Clear finishes will not hide minor imperfections in the wood. In this case, surface preparation becomes critical. With stain finishes, a wash coat of clear stain base or clear sealer may have to be applied to softwoods first to prevent rapid or uneven stain absorption. This is generally not true of hardwoods.

Masonary or Concrete Substrates

New interior masonry and concrete are prepared and finished like exterior masonry and concrete.

Metal Substrates

New interior metal substrates are cleaned like exterior metal substrates. However, corrosion pretreatment is generally not required unless the metal will be subjected to a harsh interior environment. Generally, alkyd-oil or latex-emulsion primers and paints are used on interior metal.

Synthetic Substrates

New interior substrates are identified and handled in the same way as new exterior synthetic substrates.

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Uncoated Aged Substrates

Aged substrates are harder to paint than new substrates. They may involve materials that have not been painted or coated since they were installed. An aged surface is painted like a new surface, except that the condition of the surface must be carefully analyzed.

For example, old wood has a tendency to dry out, decompose, and crack. Paint will not fill these cracks, so spackle, putty, or caulking must be used. In addition, decomposed wood must either be replaced or it must be repaired using consolidants (epoxy hardners) and fillers. Concrete and masonry tend to form a whitish powder on their surfaces that must be removed prior to painting. Ferrous and non-ferrous metals corrode as time passes, and deep pits develop on their surfaces. The pits and corrosion must be cleaned, filled, and smoothed or the paint may not cover them adequately (due to possible shrinkage) or may not adhere at all. When painting aged surfaces, good surface preparation is critical.

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Previously Coated Substrates

This type of surface can either be one of the easiest or hardest surfaces to paint. If the old coating is in good condition, it is easy to repaint with only a minimum of preparation. If the old coating is in poor condition, it may be very difficult to repaint.

WARNING! Caution must be used when removing or preparing single or multi-layered coated surfaces applied prior to 1978. These coatings may contain lead or other toxic heavy metals. Test kits should be used to determine if any layer of the coating contains lead or other toxic metals. If so, the required environmental and personal protection must be used if pressure washing, abrasive blasting, power or hand dry sanding, or scraping is used in surface preparation.

WARNING! Asbestos was used in older residential, commercial, and industrial buildings as a flocked ceiling finish, plaster fiber, acoustical tile, fire-stop board, insulation on pipes or heating ducts, asbestos-cement siding or shingles, and other applications. Do not sand or abrade these surfaces when repainting unless samples have been laboratory tested and confirmed as asbestos free. If they contain asbestos and must be abraded, special permits and special environmental and personal protective equipment must be obtained and used.

Painted Wood

When recoating exterior wood surfaces, many things must be considered. Most paints tend to chalk (form a loose powdery film) with exposure to outside conditions. Chalking is a condition where the surface of the paint deteriorates to the point where it can be washed away by rain. Years ago, chalking was a property built into almost all exterior paints because it kept the surface clean and fresh looking. Today, paints are designed to be chalk resistant.

Chalking

Chalking must be removed completely or, if this is not possible, a penetrating primer/sealer must be applied when water-based paints are used. This is because water-based paints adhere mechanically to the surface. Chalk acts as a loose coating between the original coat and the new coat, which greatly hampers this adhesion. Chalking is washed off with a strong detergent or alkaline cleaner either manually or by pressure washing. Chalking is a major problem and only one form of coating failure. Most other coating failures require removal of the old paint prior to repainting. Some of these conditions are checking, cracking, peeling, flaking, crawling, blistering, alligatoring, and wrinkling. These failures usually relate to age or improper surface preparation, mixing, and/or application. These conditions and their remedies are discussed in detail in a separate module.

Cracking And Later-Stage Flaking (Inset)

Interior painted wood surfaces are not usually subject to these failures except around windows. Generally, interior wood surfaces must be de-glossed using sandpaper or a chemical de-glosser and then thoroughly cleaned with detergent and water and allowed to dry before painting.

Painted Metal

Recoating metal surfaces involves many considerations. The surface may have a buildup of chemical salts which require chemical cleaning or removal. The surface may be rusting through and the coating may have to be partially removed. Generally, where a lot of rust is showing through on a steel surface, the entire surface must be cleaned. Other coating failures can result from improper preparation, coating selection, and/or application. If the substrate metal is unknown, the checks described previously should be used to determine the type of metal and hence the proper preparation and finish. Where coating failures are present, the cause of the failure must be found before recoating takes place.

Painted Drywall or Plaster

Recoating drywall surfaces requires proper cleaning of the old surface. If the surface is a semi gloss or gloss surface, it should be “roughed up” with abrasive paper or chemical treatment before painting. All dirt and grease should be washed off with detergent and water. If repair work is done with joint compound, it should be sealed before finished coats are applied. On rare occasions, ceilings or walls may be encountered that have a calcimine finish (a finish made of water, calcium carbonate, and glue or size). To test this finish, wipe the surface with a wet cloth. If the finish comes off on the cloth, all traces of the calcimine finish must be removed before recoating the surface with any paint.

Recoating plaster surfaces is the same as for drywall unless the paint is peeling from the putty coat or white coat (a thin, fine plaster coat over a coarse plaster base) substrate. In this case, most of the original paint and primer may have to be removed and the surface cleaned. If the surface is soft, it may have to be hardened with a white-vinegar wash. After the plaster has hardened and dried, it should be refinished like new plaster.

Painted Masonry, Concrete or Stucco Substrates

Recoating concrete, masonry, gunite, stone, fiber-cement, or stucco walls require removing efflorescence, scale, loose alkali, and flaking coatings. Blasting, power tool cleaning, scraping, and wire brushing is all used on previously painted concrete, masonry, gunite, stone, fiber-cement, and stucco walls. All voids and cracks should be filled before recoating the surface. In addition, acid etching may be used on concrete floors.

painting in Vancouver

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Wallcover Ed Substrates

Wallcoverings include any paper, vinyl, fabric, or other material pasted onto a wall or ceiling. Wallcovering should not be painted-it should be removed. If it has to be painted (often because it has been applied directly to drywall) extreme care must be used. Most wallcoverings are applied with a water-based or water-soluble glue or paste. Most latex paints contain water. Applying water-based paint to wall covering usually results in the paste loosening and bubbles developing in the paint. If wall covering has to be painted, it should first be cleaned with a mild detergent and painted with an appropriate primer/sealer. The primer/sealer will adhere to the wall covering, seal the surface, and provide a tooth for the paint to adhere to. Textured or flocked wallcoverings cannot be painted without the texture or flock showing through.

Painted Synthetic Substrates

If the synthetic substrate is rigid and the existing coating is in good condition, clean the surface and dull it before painting. If the substrate is exposed and appears to be damaged, the coating must be removed and the substrate repaired.

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Summary

Accidents are very harmful to employees and employers both, and they are often caused by poor behavior and unsafe conditions. However, most accidents can be prevented. By knowing and avoiding the behaviors that cause accidents and keeping working conditions safe, it is possible to avoid injuries and reduce hazards. The most important governmental agency concerned with accident prevention is OSHA, which has imposed requirements on trade workers designed to keep job sites and personnel safe from harm. OSHA rules apply to protective clothing and equipment, housekeeping, electrical safety, and all types of tool and machine operations. Particular hazards are presented by chemicals, ladders, scaffolds, and tools. Developing an attitude of safety is an excellent way for every worker to avoid or reduce all of these hazards.

Safety is the responsibility of each and every one of us. No one person can constantly watch and guide every operation that is going on at the job site. You should know how to do your job safely. The training you receive, complying with your employer’s safety program, and the use of everyday common sense will prevent you from being involved in an accident. An employee trying to bluff his way through a job he does not understand is just asking for trouble. Even if you think you know the correct procedures, a review may bring out an important part of the job that you may have forgotten. Don’t be afraid to ask questions. The responses you receive may help a new or less experienced coworker get answers to questions they may be too bashful to ask.

Practicing good safety attitudes means that you:

  • Report all unsafe conditions and acts immediately.
  • Keep work areas clean and orderly at all times.
  • Immediately report all accidents and injuries, no matter how minor.
  • Be certain you completely understand the instructions given before starting work.
  • Know how and where needed medical help may be obtained.
  • Wear the required protective devices when working in a hazardous operation area.
  • Do not use alcohol or drugs. If you are ill and must take prescribed medication, notify your supervisor immediately.

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