Hi all

It might be better to post this to autobody101.com, but I seem to get faster answers here. This is somewhat involved, so please bear with me:

I've been painting parts on my 69 using Dupont Variprime etch, then Omni MP 182 2K urethane surfacer then Omni MTK single stage topcoat with MR186 medium reducer. I've been having what appear to be adhesion problems with the MTK, i.e. when I've accidentally scratched a part putting it on the car, I'll have big chips come off down to the primer. When I say big, I mean 1/8 to 1/4 inch square, and they seem to come off easily. I've been applying the surfacer this way: two medium coats with a 10-15 minute flash time between, then around 1 day curing time after which I sand with 220 wet. Then I apply two more medium heavy coats with 10-15 minute flash time, one more day curing, and sanding with 220 wet, then 400 wet. I'm applying the surfacer over the etch primer and have had no problems with the surfacer adhering. For the MTK, I apply two medium wet coats with generally 5 minutes flash time (P-sheet says 5-10 at 70 degrees).

Spraying conditions are less than ideal. I'm spraying in my garage or in the driveway, and temps are in the upper 80s low 90s. I make sure not to spray in the direct sun. I'm using a small detail spray gun and a pancake compressor - not generally recommended for painting I know but it's what I have available. I take care to adjust the gun and follow the P-sheet as closely as possible, and generally the paint goes on well with a little orange peel.

I'm wondering if shooting a clear over the MTK would help make it a little tougher. I did a few test panels before I started. On one of my old fenders, I sanded the whole thing 400 wet, then painted 2/3 with MTK, and then sanded half the MTK and used Omni clear over the 1/3 of the panel with sanded MTK, and the 1/3 of the panel with sanded old finish, so I ended up with 1/3 uncleared MTK, 1/3 cleared MTK, and 1/3 cleared old finish. The cleared finish seems to be tougher than that with no clear. I'm not sure if it's because of the clear or because the substrate is old finish not the MP182.

I plan to do the other fender this way for comparison: sand the whole fender, prime 2/3 with the MP182, sand the primer, topcoat with MTK over the primer, sand 1/2 of the MTK, and clear the sanded MTK and the sanded original finish, so I will have 1/3 MTK over MP182, 1/3 cleared MTK over MP182, and clear over original finish.

Before I go to all this trouble is there something else I might be missing such as gun set up, equipment, technique, curing time or conditions, etc that might account for the seemingly weak topcoat? For instance, would letting the surfacer cure longer help? Or letting the finished pieces cure longer than one day before putting them on the car?

Thanks for listening

Cheers,
Andy

The first thing that hit me as I read your post is the heat and the small gun. If that panel is hot the solvents from the paint being applied are going to flash much faster and it looses adhesion. If you are using a little "detail" gun with a small tip it is atomizing the paint very well (you mention it is going on very well with little orange peel) this is making the solvents flash off fast as well. So with both of these in play you may end up with very little solvents to keep the paint wet while it "bites" the primer.

The second thing I wonder about is (this doesn't nessisarily have anything to do with your adhesion, but it could. ) why do you apply the primer, wet sand with 220 and then apply more? If you need to "surface" the primer for some reason, like filling scratches, pits, or blocking this is fine. But if you aren't "surfacing", there is no reason to sand and re-prime. And, if you were to be surfacing, 220 is a little too fine. Urethane primer will easily fill 180 scratches and 180 paper will CUT the surface flatter, and faster. So if you need to surface something, block it with 180 and re-prime. I personally don't like to wet sand primer. I know that a urethane should be insoluable but it just bugs me.

Brian

The first thing that hit me as I read your post is the heat and the small gun. If that panel is hot the solvents from the paint being applied are going to flash much faster and it looses adhesion. If you are using a little "detail" gun with a small tip it is atomizing the paint very well (you mention it is going on very well with little orange peel) this is making the solvents flash off fast as well. So with both of these in play you may end up with very little solvents to keep the paint wet while it "bites" the primer.

The second thing I wonder about is (this doesn't nessisarily have anything to do with your adhesion, but it could. ) why do you apply the primer, wet sand with 220 and then apply more? If you need to "surface" the primer for some reason, like filling scratches, pits, or blocking this is fine. But if you aren't "surfacing", there is no reason to sand and re-prime. And, if you were to be surfacing, 220 is a little too fine. Urethane primer will easily fill 180 scratches and 180 paper will CUT the surface flatter, and faster. So if you need to surface something, block it with 180 and re-prime. I personally don't like to wet sand primer. I know that a urethane should be insoluable but it just bugs me.

Brian

Brian,

Thank you for the input.

Don't you like how, after typing a whole page of answers, you always get more questions??

Interesting thing about the small gun and the heat. I'm generally not painting in the middle of the day, and try to keep everything out of the direct sun while I'm painting, but the ambient temp can still be in the 90s. I have noticed the topcoat drying somewhat faster than the P-sheet says. Perhaps I should try using a slow reducer instead of the medium?

Looks like it might be time to find a better gun/compressor set up. To be truthful, I wasn't intending to actually paint the WHOLE car with it, I originally was just going to do some touch-ups but like more of my projects, I ran into a few "you-might-as-wells"

Yes, I am using the surfacer to fill small imperfections. Usually after I block sand the surfacer, I've got areas where I have sanded the primer completely down to filler or bare metal. I'm also using it as a sort of a guide coat to tell me where the low and high spots are. I know people recommend using a cheap Wal-Mart type paint for that, but I seem to be finding the spots OK just by looking for where I haven't sanded off the "skin" on the top of the primer.

I'll try using some 180 between coats. I tend to err on the side of finer grit when I sand, ultimately making more work for myself in the process. I've always used wet sanding, since I'm always battling clogged paper it seems like when I dry sand. I'll give it a try on the test panels.

Cheers,
Andy

Think it's time for a new compressor and spray gun, otherwise you might end up wasting a lot of material and being frustrated.

I had to upgrade to a 2 stage 80 gallon from a 30 gallon 3Hp unit just so I could use a blaster cabinet. This hobby can get expensive with all the tools you'll need.

Yeh, But when your finished you've still got all those cool tools :) And that next project is that much cheeper :)

Trooper

I know (Mike) Newby and I don't think he'd appriciate being fumbled with. :D