The Stovebolt Site Forums The Shop Area Paint and Body Shop Epoxy Primer over really old paint & primer ?

I've read and read and I'm still very confused on this Epoxy 2K primer stuff. I'm not able to strip my 54 3600 down to bare metal all over and I cannot sand,do dent repair, prime and finish paint it all at once. I need to be able to do sections of the cab at a time. Can't I just spray the epoxy over the existing primer where I've feathered bare metal /filled areas into the old primer and painted areas? Any suggestions?

Your first misconception is Epoxy isn't 2k primer.
Epoxy is Epoxy.
2part primers are not epoxy.

If you can't strip it back to bare metal then skip the epoxy and just use 2K.

Just know going in it's (the metal) not going to be sealed like it could be.

Caso - sorry, I should have supplied more information, I'm using Matrix Systems Epoxy Black Primer MP-900 and Epoxy Activator Mav-420 so it is a 2 part epoxy primer that I got from a local Body Shop Supply Store. This may be a product that you are not familar with. Over the years I've painted 3 different cars using laquer primer and acrylic enamel and experienced no issues but I plan to keep the truck until my days are over. Maybe I'll just phospho treat the areas where I have to go to bare metal and use laquer based primer. I've never used phospho before. Any suggestions ?

I am a believer in Ospho, (that's the brand name of it). It's phosphoric acid. Once I blasted my frame, I immediately brushed on a coat of it. Turns black and acts as a primer coat. If you have a rusty area, doesn't matter. That's what it's made for.

Only drawback I have with it, once it dries, (overnight),the finish is not smooth, and it leaves a slight chalky residue. I always hand sand it with 150 or 180 grit, lightly, and then shoot a coat of primer. Smooths right out then.

I've sold it and used it for at least the last 25 years, and have always been a believer.

Mike

I'm with you Mike. Only good results with it.

Thanks Mike and Allen , good info!

Ospho is everything stated above.
It will also ruin your epoxy paint job if used as directed on the bottle.
PPG sells their own 'ospho' type product and clearly say it must be neutralized (washed off with water) while wet.
Acid prevents epoxy from curing at the molecular level.
Rustoleum and tractor paint don't care.

Hey thought I'd try to get more hands on experience with Ospho.

Does it always need to be painted over to stop rust? Or do unseen areas not need painting?

Thx Don

ospho under epoxy test/precautions

Something to consider.

If you lay Ospho on too thick, it layers up into a black coating that requires a lot of work to get it re-activated with fresh Ospho and then neutralized/washed away- so use it sparingly...

I've used very little of it on my restoration after watching that YouTube video last year. If I do use it, I neutralize it, and then wash and scuff sand everything before priming. I think that Ospho's instructions for use as a "primer" are probably sound if you aren't using it on a vehicle... just my .02

This is only a concern with Epoxy primer.

Rustoleum, Tractor paint, Arcylic enamals and oil based are not bothered by the acid.

That's why the instructions on Ospho read the way they do.

BASF, PPG and the other big manufacturing companies make their own similar product to Ospho, and all say it must be neutralized and rinsed off before applying Epoxy.

Excellent video Rick, thanks for posting. That SPI epoxy is some tough stuff. Also good testament to Evercoat Rage Gold filler. I do wonder if the first panel had been a very rusty metal like our trucks would he had a different result.

I wish we had a chemist on here to explain the different chemical reactions. My premise is that if all the acid reacts with rust, there is no acid left so it should not cause a problem. Although I've never had a problem on the farm, with tractor paint as you are fond of saying, I think issues occur when there is excess acid that doesn't react with rust and prevents epoxy from sticking. Guessing you will always have excess Non reacted acid in joints, corners, etc. I've got unpainted metal panels that have been treated with ospho (Gem rust killer is what I use), and not painted and they look exactly the same as the day I sandblasted and treated them...and it's been a couple years. To prove it to myself, the next time I fire up the sandblaster, I'm going to treat the blasted metal with ospho then give it a good wash with water and sit it beside the other non washed version to see how they age.

Ospho will remove paint.....don't ask me how I know.

Well I can remember it was a VERY meticulous process to get every molecule on one reagent to react with and neutralize its *opposite and leave nothing of itself behind.
So splash and wash is going to leave excess 99.9% of the time and why the second step of neutralization is needed. (I am not a Chemist though I play one in my garage)

On your test I suggest one piece thats ospho'd, neutralized, and then left out with the others.

I saw an HD Springer Front end forks I did a year or two ago for a friend the other day.
They sat in my attached garage for many months then in his inclosed but open and leaky shed since. All they need for epoxy prep right now is some light surface rust scuffed and removed. (YMMV)

The real test I have not seen is one between a proper PPG, SPI or BASF epoxy primer and any of the MCU (moisture cured urethane) Epoxy products. POR15 and the like.
Which has the best flexibility after it's cured?
If it bends and cracks it's going to rust. If it bends and holds it will Resist russty formations at its foundation.
Hmmm

As much as I like the epoxy, when painting a frame (if prepped correctly), I still think that brushed/rolled POR15 is the better choice. It's like a thick armor plate when it's done right. I'd have to put on many, many coats of epoxy to even come close to what two coats of the POR15 accomplishes. If it's something I'm going to sand or coat over, then no way I'm putting POR15 on it... I have no idea how flexible it is, but when I was driving out my front shackle bushings, I missed the bushing and hit the POR15 painted frame very hard with a 10 lb sledge hammer, and I could not see even a scratch where I had hit the frame!