Painting metal, does it matter if all rust is removed before?

In the context of automobile repair work, assume a metal surface has rust on it. Does it matter if all of the rust is removed before the metal surface is painted? If it does matter, then why?

It appears that at least some will claim you can do this, e.g. the company Krylon.

How I've tried to figure this out myself:

I'm assuming that the rust-covered surface area will affect how well the paint will stick to the surface. Maybe this alone can be a problem in terms of durability and sealing the metal from the atmosphere?

If it is governed by a more complex process, what process(es) should be read into further? However, I also assume that (as the aforementioned link suggests) that making the surface as smooth as possible with loose rust and debris removed will yield better results -- i.e. results that approximate that of having no rust at all there? Will it never be as good as simply having all rust removed? I know I'm asking in a way that is difficult to quantify, but "how good" will it get, compared to having all the rust removed?

After studying the Wikipedia article on rust I also imagine that from the dehydration equilibria we have that even a small area of rust can: $$\ce{Fe(OH)2 <=> FeO + H2O}$$

Even assuming the rust is properly sealed from the atmosphere it appears it can create new $$\ce{H2O}$$ under this protective layer.

But we need oxygen for the key reaction to occur: $$\ce{O2 + 4e^- + 2H2O -> 4OH^-}$$

From this how stuff works I have:

As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen.

How exactly is the water broken down into hydrogen and oxygen? From a chemistry textbook (Masterton & Hurley) I have:

For example, water does not spontaneously decompose to the elements by the reverse of the reaction referred to above $$\ce{2H2O(l) -> 2H2(g) + O2(g)}$$ nonspontaneous.

But electrolysis can certainly create these elements. But can it happen in this environment? Sealed under a surface of paint? What does the chemistry look like?

From chemical formula I have that we can get hydroxide from: $$\ce{4e^- + 2H2O(l) + O2(g) -> 4OH^-(aq)}$$

However still, from where does new oxygen get introduced into the system, assuming the atmosphere is actually completely isolated from this internal system taking place under the surface of the paint?

Should it matter, assume the environment is approximately 1-atmosphere pressure and that the temperature varies between -10 to 40 degrees Celsius.

• I have been removing most of the rust with grinding and wire brush, there are still small areas of dark color tightly adhered corrosion. I wash with soapy water, hot if possible, rinse, dry and paint. I have seen soapy water recommended by several different paint companies. It seems like the soap results in some kind of almost protective coating on the steel so it does not rust as it dries. I have always assumed that if the remaining rust is thin tightly adhered spots the paint will stick to it. I have also assumed that the rust itself is relatively inert and will not spread or further degrade Apr 20, 2019 at 20:09
• [con't] unless more moisture gets thru the paint to it. Clearly this is not as good as sandblasting to 100% white metal but it has worked for me. I have always been a little suspicious of claims of converting rust to an inert layer when it seems like it is already pretty inert? Apr 22, 2019 at 5:44
• When I worked on an offshore platform, we studied resting and painting in detail as it was a constant process there. Our sources stated that even without pin holes in the paint coating, any remaining hydroxide chemicals in the metal would pull moisture through the paint by osmosis. For that reason, we always use a neutralizing metal prep solution on the metal before painting. The phosphoric acid in the metal prep solution would neutralize any chemicals remaining in the metal. I believe the instructions said to rinse with clear water, then let dry after metal prep...
– Carl
Apr 22, 2021 at 21:06

How exactly is the water broken down into hydrogen and oxygen?

As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces -- hydrogen and oxygen.

That's extreme oversimplification. The reaction common for active metals (not including iron) is $$\ce{Mg + 2 H2O -> Mg(OH)2 + H2 ^ }$$

Iron does not react this way. However, it does react with acids, say, with hydrochloric acids $$\ce{Fe + 2 HCl(aq) -> FeCl2(aq) + H2 ^}$$

The reaction is quite slow even for concentrated nonoxidizing acids. However, in presence of oxygen and in diluted acids two subsequent reactions occur, the one noted above is followed by $$\ce{4FeCl2 + 10H2O + O2 = 4Fe(OH)3 v + 8HCl }$$

The second reaction effectively removes iron from solution, significantly fastening the overall reaction. However, even without acid present, even in slightly basic conditions iron slowly reacts with water in presence of free oxygen, forming rust. The process is hindered only at pH above 10 or at absence of free oxygen. Since natural water is usually slightly acidic due to presence of dissolved carbon dioxide, iron always rust on contact with water and atmosphere. This, actually, is a reason why concrete layer over steel frame is regulated: concrete always contains water, and surface layers quickly looses basicity due to reaction with atmospheric carbon dioxide, so steel near surface of concrete quickly rusts, increasing in volume and tearing the concrete from inside.

In the context of automobile repair work, assume a metal surface has rust over it. Does it matter if all of the rust is removed before the metal surface is painted? If it does matter, then why?

Yes, it does. Rust is weak, so any paint over it will hold weak. As long as the paint film over rusty part is broken, water and atmosphere contacts the metal and start to erode it, resulting in more paint film be destroyed. So it is critical to remove all rust from the surface being painted. Abrasive blasting, if you have the equipment, is probably the best way. While some paints may be applied over rust, it should be a very thin layer, and even then I would prefer traditional paint over cleaned surface anyway.

• So the process that allows the rust formation to continue even after paint has been applied over the rust is basically that the paint does not seal properly? In theory, if the paint does not crack, then there is no chemistry that allows the metal to continue rusting? Nov 10, 2014 at 3:05
• @AttributedTensorField if it is a proper anti-rust paint (i.e. water- and air-proof) that formed dense film and ignoring the air and water that were sealed under paint, than there is no such chemistry I'm aware of. Theoretically speaking, oxygen and water have non-zero diffusion speed even through quite dense films, so the process may continue even then, but the time of failure would be very large. Nov 10, 2014 at 5:07

Most paints will prevent oxygen and water to come into contact with the metal. Usually, rust under paint will allow the paint to break off. Some paints, like Rust-Oleum, can be applied over rust, but most cannot.

Also, with rust, you usually have an uneven surface which shows through the paint and gives an impression of a poorly done paint job.

Sometimes the best chemistry, is to simply PREVENT the chemistry of rust in the first place.

So in addition to the already well covered chemistry issues above, I think it's very important, that before coating the underlying surface, that that surface is both smooth and solid. Old guys refer to it being 'sound'.

So I've found that one important reason to remove all rust, often by sanding, is to help achieve 'smooth and solid'; in other words not to affect the chemistry of the surface, but to adjust the physical characteristics of the surface.

I've found that in actuality almost all coats of paint have tiny holes in them that show up later. These tiny holes are perhaps from bubbles or debris in the paint as it is applied. A few years goes by and tiny rust spots start to appear, then they get bigger with time.

By carefully preparing the surface each time before it's coated you can minimize the chance for these pin-holes. In other words, by again making sure the surface is smooth, so that dirt from the previous coat don't stick up, and so that the surface is clean to not give any bubbles anything to hang onto.

Multiple coats helps, but only if each new coat is again carefully applied.

BTW, another source of holes in paint where rust starts to form are places where impacts occur, like where rocks hit fenders, or where surfaces connect and expansion and contraction can break the paint joint in time.

Something you apparently missed on the Krylon webpage. "If the surface still has extensive rust after all loose rust has been removed, start with Krylon® Rust Protector™ Rust Converting Primer or Krylon® Rust Tough® Rust Fix. This primer chemically changes rust into a waterproof, paintable surface. Spray directly over the remaining rust to protect against further corrosion." Those products contain acids that are supposed to etch, seal, and "convert" the rust. Other claims by Krylon and Rustoleum are exaggerations in any climate but a high desert.

If you can't clean down to "white metal" , you will need to use conversion coating , as has been noted. It is phosphoric acid with manganese and secret stuff - depending on which brand. "Navel Jelly " is the generic conversion , I think. Conversion coatings do very well in salt spray tests but I can't remember and real numbers. However white metat is certainly the best.

Best solution is use of a first rate primer over clean metal, with all rust removed. Rust is really bad for coatings. If you've got rusting or blistering under paint, then you need to remove the paint and scrape, sand, scour, or blast off the rust. I've had both good and bad results using a phosphoric acid based liquid (naval jelly) to dissolve the rust. I tried to figure out why so much variation and I couldn't - and at the time I was working as a corrosion product development chemist! Meaning, it's a crap shoot. 20 or 30 years ago the rust conversion COATINGS were mostly just visual effects. (SO there are rust dissolvers, meant to be rinsed off, and rust conterters (made with paint resins), meant to form a primer coat (or sub-primer coat). Both are confusingly claimed to "convert" rust. How do I prepare a steel surface for painting? Remove all rust. Rinse with water. Rinse with deionized water. Rinse with acetone (gets rid of most of the water). Rinse with toluene (gets rid of most of the acetone). Dry thoroughly. Keep dry. Prime. Coat. Coat. Coat. YMMV. Could a "film forming" rust converter help? Maybe in some circumstances. Same with naval jelly. But if you can't remove all of the thicker rust, you're well and truly screwed. Chances are nothing will prolong the life of the part (or coating) any better than frequent stripping and recoating. Safety note: both solvents are flammable. Use outdoors or with plenty of ventilation (and away from heat, flames and sparks (like cell phones)).