priming over rust

depends on primer

Bill Tindall, E.Tn.

I know just enough about steel corrosion to know it is complicated. There are primers that can be applied directly to rust that is not scaly or loose. It will bond and stop further corrosion. It is done all the time in industrial situations. In the past these primers contained chromate, which I am sure they don't any longer.

Faced with painting rusty stuff it is best to seek out something labeled "industrial metal primer".

For more loose rust that may be tedious to remove, I have found that the phosphate primers work, "Skyco" for example. The phosphoric acid converts the iron oxide to black iron phosphate which is adherent and protective. Then overcoat with primer and "industrial enamel".

Hinges would be easy to wire brush first.

Re: priming over rust

Dick Coers

I'd assume the rust will first creep out around the hinge pin since the paint will fail there from friction. Then the whole hinge will have rust issues. Some kind of rust convertor would have been the way to start.

Re: priming over rust

Mike L

I use this first on the rust then prime and paint. I have never had it fail me yet. I did the inside of my 68 Mustang with this, I also do it to the underside of my cars and trucks before I paint them.

http://www.corroseal.com/

Re: priming over rust

David Yoho

It's my understanding that Rustoleum will work over the rust you described. Although they make a primer and have other products that will arrest rust, my understanding is it's not necessary. I've got an outdoor metal framed table and four chairs that has held up just fine for many years.

I think you can get to the next time you do this..

David Weaver

I've used phospho type treatments in primers and prior to them and they work well, but I have also ignored light rust quite often and just primed or painted over it. At some point, it will fail - as mentioned below, usually at edges or contact points, but I have to admit that even though many will tell horror stories, the rest of whatever I've covered isn't that quick to fail. If I were in your position, there are some little tests that you can do for adhesion (scratching, etc, to ensure that the primer is scratched off by your scratching, but not flaking off at the side of the scratch) -but I think you can pretty much finish the job and then if there's an issue with bubbling or something down the road, redo it later.

good stuff

Bill Tindall, E.Tn.

I looked it up to see what it was.....Gallic Acid. Then I looked up the corrosion science of Gallic Acid. Indeed it is a well studied corrosion inhibitor. It makes a film on steel that resists further corrosion. So, there is good science behind your favorable experience with this product.

thanks all

Jack Guzman from Maine

Painted today. Now it is only a matter of observing. I have used a product that neutralizes rust when I painted my old pickup but did not have the opportunity here. If the primer fails I will remove the hinges and do it right. Hopefully I don't have to.

Too late now, but

Bill Houghton, Sebastopol, CA

a phosphoric or tannic acid treatment before priming will convert the rust to a stable compound (iron phosphate or tannate, depending on which you use) that will accept primers.

I use Ospho (phosphoric acid base), because my hardware store carries it. I learned a couple of days ago that it also has a very long shelf life, while tannic acid based converters may have a shelf life of less than a year. Considering that I'm only halfway through my first quart, which I bought somewhere like ten years ago, the long shelf life is a good thing for me.

shelf life and chemistry

Bill Tindall, E.Tn.

The shelf live of the phosphoric acid products is forever. The product of coating it on rust is iron phosphate (black).

Gallic acid (gallic acid is a component of tannin) reacts with oxygen to become something else that is not useful for treating rust. Hence the shelf life is exposed to oxygen. Oxygen will permeate a plastic bottle. However, kept in a glass bottle, the material mentioned above should last forever so long as the paint component of the formulation is stable. The product of this treatment on rust(iron oxide) is a different form of iron oxide (magnetite) which makes a corrosion protective film on iron. It works by converting some of the ferric iron to ferrous iron to form the mixed oxide magnetite.

Rustoleum Stops Rust

Tim Greif - West of Chicago

My recollection is that the Rustoleum Stops Rust product contains some kind of fish oil that binds to the rust and creates a barrier to rust through. Just have to knock off the loose stuff first.