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Bare metal pre-painting prep

Alpine 1789

SAOCA President
Diamond Level Sponsor
The last time I painted my SV was in the 1980’s, which I’m pretty sure is before the invention of self-etching primer. If it was around then, I didn’t know about it. I learned everything I knew from reading DIY books and used a phosphoric acid solution to etch bare metal before priming. I don't know if anyone does that since the invention of self-etching primer, but need to prime the trunk lid for my SIV and am thinking it might be worth a phosphoric acid wash first.

The SIV is protected with epoxy primer but the lid came from a different car. I don't remember which one, but I had it dipped about 10 years ago to be sure it was rust-free, but it was left un-primed all this time. (Not by me, but that is a different and longer story.) The lid has been sitting on the boot and the underside still looks nearly perfect. The car has always been garaged but the top of the trunk lid had a light coating of surface rust. I’ve removed that now but there are some pits here and there (probably from before the dipping) and I want to be certain there is nothing lurking inside those pits that would come back to haunt me.

I have a gallon of this sitting around: https://www.amazon.com/Klean-Strip-GKPA30220-Gallon-Phosphoric/dp/B00945D15I, so that is one option. Is there any reason not to wash the surface with a diluted solution before priming? Alternatively, I have had good success with Rust-Oleum Rust Reformer as a general purpose primer (https://www.rustoleum.com/product-c...o/removers-and-chemicals/rust-reformer-spray/) but don't know how it would do under auto paint. I was thinking about a light coat of the rust reformer, followed by a general purpose auto primer. Any thoughts on that as an option?

Thanks. I am still a ways away from starting work on the SIV, but at least want to protect the trunk lid, especially given how close I live to salt water.
 

jumpinjan

Bronze Level Sponsor
To apply the epoxy primer, the metal needs to be well cleaned. The best way to do that is applying the Metal Conditioner (Phosphoric acid) from a spray bottle and scuff the metal with Scotch brite pad. This will clean the metal surface....AND remove the light surface rust. Wipe the sludge off with paper towels until dry. It may take a few times doing that to get it really clean.
It's not so much getting the metal "Etched"...but getting it clean. After all, your DA sander and the Scotch brite pad will scratch up the surface pretty well.
I never use the self-etching primer. Why? Well, how does the self-etching clean the metal? It must bury the rust & dirt under the primer.
So, can the self-etching primer be applied over the clean metal after its been cleaned by hand using the metal conditioner? The answer I got (and this was over 20 years ago from professionals) was...NO!!!???...
Some metal parts cannot be wiped dry and I just rinse with water and try to dry them as quick as possible.
Jan
 

jumpinjan

Bronze Level Sponsor
I use the PPG DPLF epoxy on ALL metal parts, and that includes all the suspension & engine parts.
Please stay with the professional auto refinishing brands like PPG, Glasurit, R&M, Dupont/Nason and so on.
Jan
 

jdoclogan

Platinum Level Sponsor
I concur Jan. Sand and clean-clean-clean covered by PPG epoxy primer has been my standard for years. I've never had paint bubble or lift in 30+ years. My process for bare metal is metal prep as Jan describes. Metal prep for steel or a different metal prep for aluminum. I usually use compressor air that has been filtered for moisture to dry the surface. Then I wipe down with Paint Prep solvent to insure no hand oils are left on the surface.
Note: Be certain to wear a proper mask and eye protection when painting. The fumes and atomized particles are highly toxic from catalyzed epoxy primer.
 
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Alpine 1789

SAOCA President
Diamond Level Sponsor
Thanks, guys. Just to be clear about one thing: The rest of the car has been protected with epoxy primer, but the trunk lid is currently bare steel. I just wasn't planning on mixing up and applying epoxy to it, although that is certainly an option. The car needs a fair amount of body work and my plan was to hit those areas with regular primer as I went and worry about an overall consistent surface when I was done. I was trying to avoid having to get the gun out every time I wanted to spot prime an area and then have to clean it up. I was hoping I could do that with rattle can primer.
 

jumpinjan

Bronze Level Sponsor
In my newsletter article, I cover all about refinishing work. Even though you have applied epoxy over the car, it needs to be scuffed and recoated with epoxy, because the top coating time window has expired and has hardened. The new epoxy coat will allow molecular bonding with the top coat and have good adhesion between coats.
Don't be tempted to take short-cuts....it could bite you in the ass later.
Jan
 

jdoclogan

Platinum Level Sponsor
Jan, how did you know about my ass being bitten years ago. When I was young and new everything.
 

Warren

Bronze Level Sponsor
Certainly the best thing about phosphoric acid is it's low cost.
I'm not chemist but was always fighting rusty boat parts. Especially keel bolts known for living in salt water. Firstly I'm no chemist but as I recall phosphoric acid is part of the process that is touted as responsible for changing iron oxide to iron phosphate.

As for rattle cans, heard of the two part one use ones. I have used a VHT epoxy paint and the Rust-Oleum as a base. Some of my pals use the El Cheapo Harbor freight spray guns as single use rather than spend more $ on solvents then the gun costs. I always look for free solvents at estate sales as well as R-12 for friends.

I have picked up working on car and finding missed areas in the trunk that need touch up. So again it's another useful thread here. A like button is not enough.
 

jdoclogan

Platinum Level Sponsor
Warren well done and good memory. Except you also get water. The formula is: 2 H3PO4 (Phosphoric acid) + Fe2O3 (Iron Oxide) = (reaction) 2FePO4 (Iron Phosphate) + 3H2O (water). As oppose to other acids that could rid you of the rust the Phosphoric completes its reaction and doesn't continue to react with the Fe (Iron). The phosphoric acid (5% solution) process is best for slight rust on chromed parts.
Another chemical I use is Corroseal. Here is a video https://www.google.com/search?clien...#fpstate=ive&vld=cid:24526bd1,vid:N0kP1M6UH3g. The Corroseal converts the Iron Oxide to Magnetite. You can then paint directly over the surface once it is dry. I usually allow one day.
 
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