How to Deal With Rust When Painting

Whether you find it on your tools or on the job, rust cannot be allowed to linger

It takes nothing more than for iron’s exposure to oxygen to create iron oxide or rust. And it only takes a little rust to leak through your paint job on a railing or a holding tank to make your work look very, very, bad. The three keys to dealing with rust are the same three things you use to deal with all paint applications—prep, prep, and more prep.

Prep 1: Customer prep

As mentioned, rust happens when iron meets oxygen, but it happens a lot faster if some other environmental conditions are present as well. Dampness will accelerate rusting. Electrochemical reactions between dissimilar metals close together will cause rust to occur. Salt hastens rust. Heat also makes rust happen must faster. So, before you do any surface prep, take a look at the surrounding environment and assess whether there are recommendations you can make to your customer to reduce the chances of rust coming back. Will a dehumidifier reduce the dampness? Is the heating system or the air conditioning allowing temperatures to rise too high? Is the surface you are painting made with two metals that will react with each other thereby causing rust?

There may be circumstances that will cause rust to reappear that your work will do nothing to alleviate. Imagine, for instance, that you have a rust stain appearing on a wall. Examination reveals that the stain is being produced by moisture getting to the back of an iron fixture attached to the wall. You can remove the rust stain, and repaint the wall, but the rust will reappear because the underlying problem was not dealt with. (the iron fixture was not properly under coated where it is attached to the wall).

Good investigation and informed communication with your client before you start ensures that he understands the conditions you’re working with and the limits you have in dealing with his problems. Your customer needs to understand that preventing future rust is sometimes not entirely up to you.

Prep 2: Rust Removal

You can remove the rust with sanding, scraping, wire brush or wire wheel, and/or blasting. These mechanical methods are environmentally healthy compared to chemical methods, but you end up removing good steel along with the bad and – in the case of sanding and scraping – you may not get all the rust. That may not be good for the application in question.

Sanding should be done in stages. Start with 80 grit and go up to 120 or a little higher if you need a smooth finish. Sand blasting or bead blasting is better and easier and will get into the corners, but it removes a lot of good steel. Chemical rust removal tends to get better results—phosphoric acid or a solution of phosphoric acid, alcohol will remove rust and wax and oils that can accumulate on metals. It is often used in body shops as a prep before priming to remove latent rust after sand blasting.

When you use phosphoric acid alone, it will leave a fine coating of iron phosphate behind, which actually prevents rust as well. Unfortunately, the protective layer is easily scratched and will not hold up to wear. You still need an additional application for long term rust prevention. Hydrochloric acid will also remove rust, as will oxalic acid, and they act quickly, but they also remove some metal along with the rust and they don’t leave behind a protective coating as phosphoric acid does.

Of course, there is a very big downside to these acids. They are extremely toxic. You cannot neglect any of the recommended safety procedures or disposal instructions, which means the set-up time will be longer. Be sure to wear adequate protection (gloves, the correct mask, eye protection) and read the WHMIS label that comes with the product for proper handling.

There is third method—electrolytic rust removal. In its simplest form, it means hooking up your rusty item to the negative terminal of a 12-volt bat­tery or battery charger and a piece of scrap steel to the positive pole. Then, mix one tablespoon of sodium carbon­ate per gallon of water and bathe the item in it, or scrub the rust off with a brush or cloth. Though sodium carbon­ate is still a chemical, it is much better environmentally than the acids. Still, you need to use the appropriate skin protection. Electrolytic rust removal also does not harm the good metal, so it is the best technique when de-rusting structural components.

A more recent development in rust removal is organic rust conversion. Organic rust conversion products actu­ally turn the rust into a nonrusting chemical compound. They can’t be used where the surface has started to flake with heavy rust, but, in most situations, the products are an excellent solution to rust problems. Once the covering is applied and dry, it can be a simple matter of laying down the finish coat.

The working ingredient of most of the conversion products is tannin, which converts the rust into iron tannate, a stable blue-black corrosion product. Gent and then rinse. That’s because the primer must be in direct contact with clean metal for proper adhesion. Grease, dust, and dirt must be removed, or corrosion will occur under the primer and lift the paint. And if you get flash rust after the rinse dries, be sure that you wipe it off with a cloth dampened with paint thinner.

The primer and finish coats for metal come in all the bases: epoxy, enamel, latex, and polyurethane—even alumi­num. Make sure the product you choose is right for the application, and the primer is right for the finish coat. If you decide to spray coat your finish coat, you should still work the paint in with a brush after spraying to ensure the paint gets deep into every small pore.

Each of these methods of rust removal and prep has pros and cons that make one method better than another in a given situation. Sand blast­ing may not be possible if you are refinishing iron pickets in a $1-million dollar home full of priceless art. The acids may not be acceptable to your client because of their toxicity. Again, communication with your client and a thorough knowledge of all your options is the best antidote to coating failure.