Avoiding Sanding Scratches with a High-Sheen Conversion Varnish

Advice on sanding and finishing methods to prevent sanding scratches from telegraphing through high-sheen finishes. February 19, 2006

I'm trying to spray Chemcraft's pigmented conversion varnish, Plasticolor, at a 90 sheen. It's on MDF that was sanded to 220 grit, coated with two coats of primer (plastiprimer), and then sanded with 320 no load paper. All the sanding marks from scuff sanding the primer coat are telegraphing through when the finish dries down. I catalyzed 10% and reduced it down about 10% to get the proper flow rate of 20 seconds in a Ford cup. I used a slow reducer to prevent air bubble entrapment. Also, I sprayed two coats (1 good box coat). Since this is such a high sheen, every little scratch shows.

Forum Responses
(Finishing Forum)
From contributor A:
I haven't tried this with Chemcraft but it works with other products: do all of your filling and sanding on the first primer coat. Lay down a smooth second primer coat and don't sand it. No sanding - no scratches. Put your topcoat on within a couple of hours and it should look smooth as glass.

From contributor B:
I have used Chemcraft for 3 years now and never had problem with sanding marks after topcoats. I agree that you do all the filling and sanding after the first coat, but you need to at least light scuff it after final coat for the lacquer /CV to hold. Why don't you try 400 grit sandpaper? Chemcraft 90 sheen is pretty thick. I usually use 10% cat., 20-30% thin., 5% slow red. I'm not sure what color your plasticolor is, but you might be using color that is way off with your plastiprimer. You can also try to tint your primer closer to your final color.

From contributor C:
Do you want high gloss? Sand with 400, 600, then maybe even wet sand with 1000 for a nice finish. To fix your current problem without starting over, wet sand with 400, minimal burn through, spray one light wet coat of color, and allow to dry. Wet sand with Abralon 500, 1000 grit de-nib pad. Don't sand through color, clean thoroughly and spray one coat 90 sheen clear. Then base coat/clear coat and it will look like glass.

From contributor D:
Scratches that telegraph thru the topcoat from underneath the film may be caused by either sanding the coat underneath it too hard, too soon, or from sanding with a paper that is tearing the film. Possible solutions are letting the finish dry a bit longer prior to sanding, changing sanding paper to a less aggressive grit, or following up the sanding process with a Scotch brite pad to minimize the scratch prior to putting the final coats on.

From contributor E:
Your problem is not with the coating, it's with the sanding process. This is how I spray Chemcraft conversion varnish on MDF. Sand the entire door with 220 grit 3m gold stick it 255L. Spray one coat of plastiprimer catalyzed with no reduction. Cure for 4 hours under general conditions, or one hour if baked at 150 degrees for 10 minutes and 5 minutes of flash for a total 1 hr. and 15 minutes. Sand with 320 255L and spray again with a box coat of unreduced primer. Repeat the curing process and sanding process. Spray 1 box coat of unreduced top coat and there shouldn't be a printing problem. I would check to see what the throw is on the orbital sander, what the rpm rate is on the sander, and the size of the paper. If the throw is larger than 3/32 this is what the problem is. If the throw is larger than 3/32 then drop down to a 5 inch pad. This will allow you to sand with less swirl marks. You may need to reduce all the materials at 20% if you’re not running heaters. Remember that bad sanders and cheap sandpaper equal rework. I use Chemcraft CV products, but it wouldn't be my first choice.I'd use pigmented polyurethane and only spray one coat of primer sealer and a top coat.