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 you’re wasting the extra cost of these paints if you paint them on a surface with rust. If the ad or literature doesn’t specifically say you can "paint on rust," then you have the wrong product.
Treatment vs. Conversion
It’s 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 don’t know why you want to choose a product that requires washing off, if other products don’t 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.
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 you’re not a chemist, it’s 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.
http://www.zerorust.com/tech.htm#prep
http://www.por15.com/
this has been described as "a single stage di-isocyanate based paint," anhydrous, and may be a moisture curing urethane.
http://www.rust-preventative-paints.com/paint_over_rust/index.html
http://www.masterchem.com/index_hammerite.html
http://www.krylon.com/product/op_rust.asp
http://www.directchem.com/056ruscon.html
http://www.loctiteproducts.com/Loctite_Products/RustTreatmentCleaners/NavalJellyRustTreatments.cfm
several Loctite products including Naval Jelly.
http://www.rust.co.uk/
Paint company that claims success painting on rust, but they’re in the UK.
http://www.slickote.com/new_page_11.htm
a washprimer acid etch product
http://hirschauto.com/miracle.htm
a moisture cured urethane product for auto repair
http://www.benjaminmoore.com/im/im-specialty.html#M84
rust pretreatment
http://www.benjaminmoore.com/im/im-specialty.html#M82
rust converter coating
http://www.sherwinwilliams.com/industrial-marine/products/coatings/polyurethane.asp#
(select Corothane I Pre-Prime, I can’t get the direct web address)
Here are some rust resistant primers, but they are NOT for painting over rust, they’re for painting on clean metal.
http://www.paintinfo.com/mpi/approved/mpi079.htm
alkyd based
http://www.paintinfo.com/mpi/approved/mpi020.htm
epoxy zinc based
http://www.paintinfo.com/mpi/approved/mpi023.htm
supposedly surface tolerant, but preferably used on clean metal, and no mention of rust tolerant.
http://www.paintinfo.com/mpi/approved/mpi107.htm
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
http://www.tfhrc.gov/hnr20/bridge/mastic.htm
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).
http://www.halox.com/corrosioninhibitors.htm
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:
http://www.paintinfo.com/mpi/approved/mpi031.htm
And a positive review of "MC" urethanes at
http://www.tfhrc.gov/pubrds/novdec98/laboratory.htm
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
http://www.rustoleum.com/category.asp?SubCatID=53&MainCatID=1
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 haven’t 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 don’t know how good "cold galvanizing" can be. But look at
http://www.sherwinwilliams.com/industrial-marine/projectprofiles/bridge/masterpiece.asp
And, I have seen very few zinc paints that claim they can be painted over rust. Although here’s one that shows a picture of it being sprayed on rust.
http://www.lpslabs.com/Products/CorrisionInh/ColdGalvanize.htm
Here are some articles, long and short, and miscellaneous links, some about rust-resistant paint, some about painting on rust, (remember the difference!)
http://www.abrn.com/ARCHIVES/0699tech.htm
http://www.classicar.com/articles/restoration/rust/part2.asp
http://www.mirafiori.com/faq/content/rust.html
http://paintstore.com/archives/coat0600.htm
Here are some interesting links concerning paint in general:
http://www.paintinfo.com/mpi/approved/prodtype.htm
an interesting list of the types of paint products.
http://www.pcimag.com/CDA/PCI100/companyprofile/CompanyProfileIndex/0,1855,1,00.html
A list of the largest paint manufacturers.
http://www.paintinfo.com/mpi/guide/repaspec.doc
a complete painting specification. See paragraph Applications .6 near the end for some recommended paints for steel.
http://corrosionsource.com/links.htm
a website with many corrosion related links.
Here are some mostly short opinions I ran across:
http://www.nacecorrosionnetwork.com/archive/9812/0251.html
a short opinion about rust converters, says it is phosphoric acid and resin.
http://palimpsest.stanford.edu/byform/mailing-lists/cdl/1998/1560.html points out that naval jelly must be removed, not left on.
http://www.ytmag.com/ttalk/messages/29127.html
why rust converter = phosphoric acid wash works.
http://www.ytmag.com/ttalk/messages/29115.html
short opinion recommends surface work and rust converter.
http://www.ytmag.com/ttalk/messages/29183.html
opinion on epoxy coatings says power wash first.