Sanding Grits, when Epoxy is used as high build primer

craig429

Member
I am using epoxy as my high build primer over poly primer. Before spraying epoxy over bare metal, the metal was sanded with 80 grit. Before spraying epoxy over other primer, it was sanded with 180 or coarser.

At this point, I think I have enough epoxy (older than the 7 day recoat window) on the car to perform final sanding, 320 grit dry, then 400 grit dry using a hard block, and finish with P600 grit wet using a soft block.

If I find out, I am wrong about having enough epoxy to finish sanding and need to spray a couple more coats of epoxy. Do I need to sand the epoxy back to 180 to have enough mechanical bond, or can I leave it at 320,400, or even P600 grit?
 

strum456

Oldtimer
Adhesion of epoxy on epoxy with a 600 scratch is no problem. I think you might be jumping into the fine paper a little too fast though. Epoxy works well as its own guide coat. Maybe start on it with 220 if you're pretty confident the poly is perfect. 180 would be a safer bet.
 

theastronaut

Promoted Users
Rough paper shapes, smooth paper smooths. You'll need a rough enough paper to cut down the texture and wave from spraying epoxy even if the poly is perfect. I'd start with 150 to flatten the primer really well, then spot in any break through areas and reblock those with 150. Then cover every square inch with dry guide coat and block with 320, then 500. It's better to get the epoxy perfect and sanded up to 400 grit, then do a coat of thinned epoxy as a sealer. Be extremely careful when wetsanding anything with poly primer, it's not waterproof and any break through will need to be stripped and baked to make sure there's no trapped moisture, or there will be pimples later.
 

Chris_Hamilton

Trying to be the best me, I can be
If you are new to this it's a good idea to get everything as close to perfect in 180 before moving up the grits. Most common error I see guys with not a lot of experience make when blocking is not blocking a panel enough, using too fine a grit, then applying more primer. Often repeating that cycle several times. So you get a lot of millage build which is not what you want. If you stay at 180 till you have all your lows blocked out then apply a couple more coats and finish with 320 dry then 600 wet, you help avoid some of that. Trying to correct a body with a grit higher than 180-220 (max) is a mistake IMO. Finer grits like 320 and above purpose are to remove and refine the sanding scratches, not to correct the body.
 

craig429

Member
Thank you for the comments. I have only painted 3 over-alls and a few odd pieces. This is my first full restoration that needed lots of metal work including lots of panel replacements due to rust. I have learned that sanding poly with sanding blocks made out of acrylic using 80 grit show more low spots than using a Durablock (I am not pressing hard, I am letting the sandpaper do the work). And sanding epoxy with acrylic blocks with 320 can cause pigtails/ sandpaper clogging versus using a Durablock. I think the acrylic block is so hard that the sanding dust can't escape where as the Durablock is softer which allows the sanding paper to move around and the sanding dust to not collect on the sanding paper. It does help to make sure the epoxy is cured by using an IR heater, as I was trying to sand 4 day old epoxy.

I sanded the poly first with 80 grit and after a couple applications of poly and sanding, the final poly was finished sanded with 180 grit with all guide coat removed. I sprayed the whole car with epoxy. Since the car was flat, I thought I would first sand it using a hard acrylic long block with 320grit dry and end up with 600 wet using a softer short block. When I started sanding with the acrylic block with 320, I could see where there were a couple slight low spots. These spots sanded out with 320. This is where my original question came from, if I find out that I needed to spray additional epoxy, would I need to sand it coarser with 180 or would 320 give enough bite.

theastronaut, If I do hit poly while wet sanding, are you saying the poly will need to be sanded off and start over, or will letting it sit for days maybe using an IR heater on remove the moisture?

After I started using poly, I learned that it soaks moisture which I think could cause an issue with rock chips. I am now not sure if using poly was the right choice.
 

theastronaut

Promoted Users
theastronaut, If I do hit poly while wet sanding, are you saying the poly will need to be sanded off and start over, or will letting it sit for days maybe using an IR heater on remove the moisture?

Yes, the moisture must be removed or it'll cause big problems later. I'm switching from Slick Sand to Clausen's All-U-Need since it's poly that is waterproof.
 
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