Sandpaper scratches

W

wrndln

Well-known member
I think I made a mistake when I sealed (DP40) and primed (K36) my car. I used 80 grit sandpaper to shape the filler like most people do. I then applied the sealer and the sanding primer thinking the K36 would "fill" the scratches. Now after 3 or 4 coats of K36, when I use the grease and wax remover, I can see the sandpaper scratches. I think the sandpaper scratches (undoubtedly from the 80 grit) will show if I paint the car with the scratches present. The surfaces seem quite smooth, but you can see the scratches when a glossy liquid like water or grease & wax remover is applied. Next time I will use 150 or 180 to sand out the 80 grit scratches before priming. In the mean time, if I apply a few more coats of K36, should the scratches disappear? Would something like Evercoat FeatherFill high build primer hide the scratches? I don't know if FeatherFill will work OK over K36, as someone told me it shouldn't be used over K36. Any thoughts? This is the first model A I have painted and this is a rookie painting mistake I won't make again. Thanks in advance for any help with this problem.
 
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Rather than apply more product I would block the surface down with a finer grade sand paper, say 240 grit to remove the heavy scratches and then re-prime.

This way you will avoid thick film build up and sink back later.

The extra block sanding will also help in getting your filler repairs straight.
 
I'm with Spider here,keep sanding instead of adding.I don't know how you mixed it,but K-36 can be made to work like Featherfill,and four coats can be too much.You have plenty to work with,and the more you block the better your job is going to look.I used a lot of Featherfill in the late 70's and 80's,and most of it has come back to haunt me now.I really should have known better when 3 out of 5 cans would have to go back to the paint supplier because it was hardening up in the bottom and could not be mixed properly.The stuff seems to shrink up in a few years.I put some on a T about 1980 or so,now the car is starting to alligator.It wants to stick,but the Featherfill is shrinking,and it is pulling itself together in 3-4 inch patches.That car has DP-40 on it,and it looks as if I can sand off the featherfill and finish and leave the DP-40 to go over again.
 
I always sand the primer/surfacer down to the sealer before recoating with surfacer. I think what you are seeing is the porosity of the filler rather than actual scratches.
 
better safe than sorry later. block sand more and lose those scratches. they most likely will be seen if not. 80 grit is to coarse.
 
80 grit is what I use to remove paint and excess filler. The trick is to use a finer grit on primer to remove the scratches (120-220-320-400-600) in that order. I would not strip and redo if the primer/sealer is on ok. just work the scratches out. If you get down to bare metal or bondo just re-seal and prime. I final sand with 600 grit before paint.
 
I am a PPG certified Tech, 80 grit is fine for filler although I usually use a finishing filler and finish it off a little finer.
4 coats of properly applied K36 will take care of 80 grit.
You posted nothing about sanding the surfacer (K36), you do know that you MUST sand primer surfacer, right???

You will see the 80 grit scratches after priming, you need to guidecoat and block with a finer grit depending on the paint you will be using, usually 400-600 grit. This sanding levels the surface and the 80 grit will be filled.

As other said, always sand again before adding more product. Once you satrt hitting alot of bare metal it's time to reapply K36.
 
I'm not a paint tech just an average guy who uses all kinds of different paint products. When you fill 80 grit will work fine for the rough out. But I sand the shape as close as I can working through grades from coarse to finer. I start with 36 or 40 while the mud is still semi soft. You will burn through alot of paper but it gets through the gooey layer faster. Move to 80 then 220-240 then onto 320 and finally finish with 400. Then prime lightly. K-36 is great stuff but man it's spendy. Primer applied too thick will shrink over time just like Filler and that sucks to have a car good for a year and then need to be re-done because it's finally drying out underneath. Then 4-600 the K-36 and seal and scuff and paint.

Treat the base materials like they are the finished product every time and the ending paint job will be nothing short of spectacular. Paint is just a coating which is not meant to hide but to show off the underlying work. always use the best and longest block you can and it'll be even better.

Good luck and have fun,

Tim
 
Beginners have alot of trouble properly sanding body filler to contour when using finer than 80-100 grit. It's not impossible to go finer, just much more difficult for a beginner with out messing up the shape.
Sanding with 80 makes it easier. A few swipes with a finer grit to knock the tops off the 80 grit scratches will work, but stopping short with 80 and finish contour sanding with a finer grit increases the skill requirement.

Sanding filler to 400, priming and sanding the primer with 400 is not gaining much.
 
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