How Many Coats of Primer?

Can you get away with using just one coat of primer under conversion varnish, when applying with an air-assisted airless sprayer? December 8, 2006

Our shop has been trying to lean up on our time in the spray booth. We upgraded our equipment to a CAT air-assist and upgraded our coatings to CV (clear and whites). Today was the first day I used the CAT aaa with primer (Clawlock). We were told by the sales folks and technical support from MLC that this combination was a two-coat deal - one primer and one topcoat (Resistant or Stealth CV).

The material was plywood (D3 maple) and 5-part flat panel doors. I sprayed on a solid coat of the Clawlock, and had to thin it about 15-20% to get it to lay down properly. After looking at the primed items, I still see a lot of wood grain (maple hardwood and birch/maple ply). I haven't sprayed the topcoat yet (waiting on customer dragging their feet).

If this was MDF, I would have polished the route with 400 grit, which has worked with a two-primer-coat schedule using Magnaclaw. I've read folks here claim to get away with one-coat primers that look good. I can't see how. Do you just spray the peanut-buttery thick primer as it is in the can, laying on about 8 wet mils or more to get it to lay down? How do you pull this off on vertical surfaces like bookshelves? Part of our goal in this upgrade was to eliminate a step in our standard jobs (and I know the clears are two-coats because I've done it with a loaner AAA unit). But how are you doing this with primer and whites?

Forum Responses
(Finishing Forum)
From contributor S:
The problem will be the coating dropping into the pores of the hardwood veneer. The spray gun applies a consistent coat of 100-200 onto the surface. This then drops into the pores, filling them, but leaving dents in the paint finish. This is your open grain look in stains and varnishes. The only way around this is to use a grain filler, either before the primer, or between primer and topcoat, or use MDF (no one will know the difference once painted).

From contributor A:
If you spend more time prepping and sanding, you might get away with one coat of primer. Also, if you do two or three passes, it helps. We used to paint maple ply, but you can't get it as flat as MDF no matter how many coats you spray. We switched to MDF and the one coat primer works fine. It even looks better than three coats of primer on ply.

From contributor R:

When doing a paint grade job, I first apply a vinyl sealer coat that's thinned down a bit. It's then sanded very well with 280 3M silicone carbide paper. I apply two coats, one right after the other, of Sherwin Williams low VOC primer. When that's dry, I sand it with 280 3M paper and apply my opaque coating. I find that sealer sands easier than the primer, and since the sealer is smooth after I sand it, the primer coats over the sanded sealer require a good but not so aggressive sanding. Some finishers like to apply a watered down glue mixture to all end grain of plywood and MDF prior to priming.

From contributor B:
Personally, I think the Clawlock should work for you. I just think you thinned it too far. I usually spray my Clawlock thinned about 5%-10% with the Flow Enhancer or Care Reducer (depending on heat). Then give it a good 4-5 wet mils and there you go... a good one coat primer.

From the original questioner:
Thanks for all the info. After spraying all the primer (the first coat), they just didn't look like one coat would hide the grain enough (we shoot for a level of finish higher than what our competitors in the area seem to, but they all tend to use CAB lacquer, so it's not hard).

Anyhow, after deciding to do a second coat on all the doors and on open cabinets, I noticed that after sanding the first coat, everything looked pretty good - better than I would have expected. I sprayed the second coat pretty thin, since they didn't need another heavy coat and they look great. I would have likely been alright without the second coat. I think the biggest problem we have is that the center 1/4" panels we used were a Baltic birch that tends to be a lot fuzzier than the 1/4 maple we get, but the Baltic birch from our suppliers is a true 1/4", which actually fits in the dado of our door rails/stiles a lot better than the maple. The birch is just so fuzzy and grainy compared to the maple.