SPI Primer on rusty metal


Promoted Users
Hi all
I was always told that the panel has to be clean, sandblasted and 100% free of rust, before applying epoxy. I agree 100% with that and I'm not questioning it. However, about a year ago, I painted some engine parts with SPI epoxy and had some leftover. So I decided to do an experiment. What I did was take a rusty piece of metal measuring 12"x 12". I rinsed it with water and used 80 grit sandpaper to remove 50% of the rust from it. So the panel was still very rusty but not flaky. Then I applied Ospho to it and once it dried, I neutralized and cleaned it with acetone. After that, I applied two medium coats of epoxy on it. Now, it has been 15 months since then, and the panel shows no signs of deterioration on the epoxy primer. As for the location of the panel, it has been sitting outside in a shaded area but has been exposed to some water from rain, etc.

I know that a lot of people are against treating rusty metal with acid instead of sandblasting it to make it shiny like a mirror before painting, but sometimes you have to take the easy route. With that said, is 15 months too soon to start showing problems with adhesion?

As far as the adhesion is concerned, it is still very solid and hard.

As someone new to painting and doing it as a hobby, I would like to know what is wrong with what I did?


Mad Scientist
You can’t determine adhesion by the way it feels.

Sometimes encapsulating rust can actually exacerbate the rust since water and oxygen within the rust are now trapped and can’t get out. They will continue to react until the reaction has reached an equilibrium and one of the reactants is completely exhausted.

Sounds like an interesting experiment though. Just keep letting it go.


Promoted Users
To the original poster--nothing. Provided your surface was actually dry enough to be successful. Rust (iron oxide FE2O3 and others) is not corrosion--it is the byproduct of the corrosive attack of water and FEOH- at the inter-face of any moisture with dissolved ions in the water on steel surface. The weak acid wash with Ospho or equivalent neutralizes the caustic attack. Flush off any weak acid not used to neutralize the FEOH- and you have a good foundation for any primer provided you have heated the surface adequately to drive off any moisture remaining and long enough for any to flash off. In industry, painting over rust is done every-day--that is done with flame-scaling on a lot of large structural steel pieces. You heat to about 200F with parts moving on a conveyor or chain under an set-up. No one uses epoxy for such and still has success for decades. You can too.


Promoted Users
you left rust , let ospho dry , sooner or later it will fail.
well I did say that I neutralized it. I didn't say how, because there was a procedure by SPI as how to neutralize the Ospho.
And yes, I left rust on purpose to see how it fail. But the damn thing hasn't failed after 15 months LOL