Paint Over Rust

originally posted in 2001, revised June 2017

Good link 6/17/17, search Google for "Adhesion to Rust"

Good link 6/17/17, search Google for "Bridge Painting Specification"

Good link 6/17/17, Current Standards of SSPC Society for Protective Coatings, scroll to the section on paints, or browse the whole website

Most of below is from 2001.
Keep in mind you want "Paint over Rust." Much of this is "paint on steel" which is not "paint over rust."

Paint on iron or steel usually has a limited time before problems develop, such as the paint coming off and then rust forming, or rust forming under the paint eventually breaking through the paint.

Time is a factor. If the paint job does not have to last a long time, you do not need to concern yourself with rust issues. If it means something to you that the paint job last years and years before needing any further attention, then do concern yourself with rust issues.

Rust-Resistant paint vs. Surface Tolerant paint:
If you want to paint on rust, you want a "surface tolerant" paint to go directly on the rusted surface. There are plenty of "rust-resistant" paints, but most of these require that you put them on an absolutely clean, bare metal, no-rust surface, or a perfectly clean, properly primed surface. So youíre wasting the extra cost of these paints if you paint them on a surface with rust. If the ad or literature doesnít specifically say you can "paint on rust," then you have the wrong product.

Treatment vs. Conversion
Itís hard to pin down whether there is a difference. I would say a treatment is a product that you apply and wash off. A conversion or converter is a product you apply and it stays on. Both of them seem to contain phosphoric acid, and change the surface of the rusted steel to a black colored iron phosphate. I donít know why you want to choose a product that requires washing off, if other products donít require this. Maybe the treatments are stronger acids that have such an excess of acid that total reaction is assured and the excess must be washed off, while maybe the leave-on converters are weaker acids that are fully consumed by the reaction (but perhaps there was not enough to reach all the substrate molecules that needed converting).

Major industries that are concerned rust-resistant and surface tolerant paints are the auto repair industry, the outdoor sign industry, and the bridge maintenance industry. The paint problems of these industries are most often the examples used in "paint on rust" and "rust resistant paint" research. Note that you use much less paint on an auto repair than on a bridge, and so the auto repair paints can be expensive while the bridge paints must be more reasonably priced.

Two observations about paint:

Unless you are a major professional buyer, you buy paint on faith that you know what you are doing, or on faith that the salesperson has good knowledge and advice. You have very little evidence that the product that is right there in front of you, will give you the results you want.

Paints are mixtures, and every manufacturer will have different proportions in their mixtures, so it may be difficult to compare any two manufacturers "apples to apples."

Reputation of the paint manufacturer:
The quality of the "paint on rust" product is hard to determine unless you are a specialist in paints. Beware of ALL products that claim to paint over rust, even if the claim is loud and confident. Just about any paint looks good when it first goes on. How do you know it will do what it really claims? Maybe they have 1 percent of a superior ingredient, and your paint job will last 1 percent longer than a regular paint. When buying products that depend on chemistry, and youíre not a chemist, itís probably wise to see what the major manufacturers offer before going with a previously unknown brand.

The above said, here are some companies that offer paint on rust products.
Bad link 6/14/17 goes to Rustoleum page not found - Rustoleum advertises that this is a primer to paint directly on rust.
Bad link 6/14/17 goes to Rustoleum page not found - The spray version of the above.

Bad link 6/14/17 an advertising link site
Good link 6/14/17 this has been described as "a single stage di-isocyanate based paint," anhydrous, and may be a moisture curing urethane.
Bad link 6/14/17 page not found
Link now goes to 6/14/17
Bad link 6/14/17 goes to Krylon page not found
Good link 6/14/17
Bad link 6/14/17 goes to Loctite file not found several Loctite products including Naval Jelly.
Good link 6/14/17 Paint company that claims success painting on rust, but theyíre in the UK.
Good link 6/14/17 a washprimer acid etch product
Bad link 6/14/17 goes to Hirshauto page not found a moisture cured urethane product for auto repair
Bad link 6/14/17 goes to Benjamin Moore page not found rust pretreatment
Bad link 6/14/17 goes to Benjamin Moore page not found rust converter coating
Bad link 6/14/17 goes to Sherwin Williams search page (select Corothane I Pre-Prime, I canít get the direct web address)

Here are some rust resistant primers, but they are NOT for painting over rust, theyíre for painting on clean metal.
Bad link 6/14/17 not found alkyd based
Bad link 6/14/17 not found epoxy zinc based
Bad link 6/14/17 not found supposedly surface tolerant, but preferably used on clean metal, and no mention of rust tolerant.
Bad link 6/14/17 not found water based

Glossary and Definitions:
Acids - Phosphoric Acid, Chromic acid, Tannic acid. All used with some success to react iron to become part of a molecule that is not rust, and the iron is therefore not available to become rust.

Coating - technical term for paint.

Conversion Coatings, "Converting," "Converter" - A primer that includes chemicals that react with the steel substrate to change some of the surface iron to another chemical, typically iron phosphate, to prevent the formation of iron oxide.

Epoxy mastic - a paint that is successful on steel, but not necessarily if applied on rust. Epoxy mastic has an advantage of covering well with only one coat. However, see a negative review of it at Bad link 6/14/17 goes to homepage

Etching - putting an acid on a clean metal surface and then rinsing it off with water. The surface is corroded in a desirable way. The entire surface is now uniform. If there were various chemical compounds bound to the surface previously, they are now gone, there is now only one chemical on the whole surface. For example, etching clean steel with phosphoric acid leaves the entire surface with a uniform layer of iron phosphate. This temporarily prevents the formation of iron oxide on the surface.

Inhibitors - chemicals with properties that block or reduce the speed of corrosion. For more info, see this company that makes them (but only sells them to paint manufacturers). Bad link 6/14/17 goes to Halox page not found

Iron oxide - a molecule of rust.

Moisture Cured Urethanes - a paint that is successful on steel, but not necessarily if applied on rust. A list of such products: bad link 6/14/17 not found And a positive review of "MC" urethanes at Bad link 6/14/17 goes to homepage

Naval Jelly - now a brand name of Loctite, characterized by containing phosphoric acid.

Pre-treatment - usually contains an acid to convert any iron to compound other than iron oxide. Usually wiped off after a short time allowed for the reaction.

Primer - the first coating put on the substrate. It should be formulated to stick to the substrate better and for a longer time than ordinary paint.

Rustoleum - a brand name of Rustoleum company, characterized by containing fish oil. See Bad link 6/14/17 goes to Rustoleum page not found

Solvent-Borne - Paints are generally either solvent-borne or water-borne, the liquid that makes the paint stick and spread on the surface, then it evaporates. Solvent-borne paints have a reputation for lasting longer on steel. Solvent-borne paints have the disadvantages of being intoxicating to breathe and causing air pollution. The common solvent-borne paint is "alkyd." The common water-borne paints are acrylic and latex. Acrylic can be successful on steel. Latex does not have a good reputation on steel.

Substrate - the surface you will start to paint on after you have removed all old layers of paint and cleaned the surface. It is assumed your substrate is bare clean steel, but if you havenít removed all the paint and dirt, then that is still called your substrate.

Surface Tolerant - coatings that are successful when applied on surfaces that have rust or are otherwise not recommended substrates.

Tight Rust - a surface that has been cleaned fairly well, with all loose rust scraped off, and maybe even sanded, although not sanded down to bare clean steel. Many products that claim to "paint over rust" state in the fine print that they mean painting over "tight rust."

Washprimer - another name for a conversion coating, or a pre-treatment, which is different. Check whether it has to be washed off or not.

What about sacrificial zinc? Galvanizing puts a layer of zinc over clean rust-free steel, and then resists rusting very well. In theory, putting zinc rich paint over steel should greatly slow down the rusting process. However, galvanizing is done in a "hot" dip process, I donít know how good "cold galvanizing" can be. But look at Bad link 6/14/17 goes to Sherwin Williams search page And, I have seen very few zinc paints that claim they can be painted over rust. Although hereís one that shows a picture of it being sprayed on rust.
Bad link 6/14/17 goes to Lpslabs page not found

Here are some articles, long and short, and miscellaneous links, some about rust-resistant paint, some about painting on rust, (remember the difference!)
Bad link 6/14/17 goes to an auto body links page
bad link 6/14/17 goes to Classic Car page not found
Good link 6/14/17
Bad link 6/14/17 not found

Here are some interesting links concerning paint in general:
Bad link 6/14/17 not found an interesting list of the types of paint products.
Bad link 6/14/17 goes to PCI mag page not found,1855,1,00.html A list of the largest paint manufacturers.
Good link 6/14/17 a complete painting specification. See paragraph Applications .6 near the end for some recommended paints for steel.
Good link 6/14/17 a website with many corrosion related links.

Here are some mostly short opinions I ran across:
Bad link 6/14/17 goes to a log-in page a short opinion about rust converters, says it is phosphoric acid and resin.
Bad link 6/14/17 goes to a conservation/preservation website points out that naval jelly must be removed, not left on.
Good link 6/14/17 why rust converter = phosphoric acid wash works.
Good link 6/14/17 short opinion recommends surface work and rust converter.
Good link 6/14/17 opinion on epoxy coatings says power wash first.

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