D.A. Marks

[Paul Clark]

The guy who is lined up to paint my car just finished a friend's Vette. If you look closely, you can see the sanding marks made by the D.A..
I've done just enough body/paint work to be dangerous. At what stage should I sand on my car to make sure I don't get the same marks. Bet it's on the primer (?).
If so, what grit will remove the marks but still leave enough "rough" for the base coat to stick? Wet or dry?
Thanks

 

[John Larkin]

After bodywork, the primer goes on for blocking. Some bodymen can block with a DA but others can't. Bondo shrinkage usually causes DA marks to show also. Bondo is body filler but it is not a substitute for poor body work. It should never be more than 1/8" thick IMO. Allowing it to fully cure is critical, the longer the better; at least a week for something other than production work. A flat block or longboard is usually used on primer. It's a workout. Pretty much you start with 180 dry on primer and a guidecoat to find low spots and check one last time to see if more bodywork is needed. If none is, then reprime and again after each change of paper grade. Start blocking with 320 dry, then 400 dry and finally 600 wet. You are basically taking all the primer off each time except for the minimum needed to cover the body. Primer should only build up in scratches and minor low spots, it should not be considered "body filler". Afterward apply a coat of sealer and you are ready for paint. You don't sand sealer but you should tint it to your final color coat.

 

[Paul Clark]

Thanks, John. I really appreciate the info.
The car in question was a 'Vette, and there wasn't any bondo.
The shooter used a guide coat.
Think I might show up after he primes and guide coats it and do my own sanding. Then I'll "suggest" that he primes again. I don't like those little circular sanding marks.
This ain't a white Corvette. ;=)
Didn't know that the sealer goes on after the primer. Thought it would be put on before. Live and learn.
Thanks again.

 

[John Larkin]

Sealer can be sprayed first for bare metal protection. However that is sanding sealer. Top coat sealer is to hold the primer and bondo down. A week-long cure after block sanding lets the chemicals dry out of the bodywork and helps guard against shrinkage and those darn sand scratches.

Block sanding should be done in an X pattern. This avoids divots and gouges. Try to never sand in a straight line. It also compensates for panel curve.