Another question is should the surface be dry or wet sanded? The best thing to do is to follow the manufacturer's recommendations but generally wet sanding polyester primers is not suggested. These materials may allow moisture to penetrate the surface, which can create problems later on. In our case Jake Brazille, of Jake's Place in Florence, Oregon, applied DP90LF epoxy primer followed by D839 Primer Surfacer, which may be dry or wet sanded, depending on preference.
For D839 PPG recommends the following:
For dry sanding: 240 followed by 400-500 grit
For wet sanding: 400 followed by 600-800-grit.
We found that D839 dry sands extremely well without clogging the sandpaper so there was no need to make the mess that comes along with wet sanding, at least not yet.
Our first round of block sanding was done with the longest blocks that would work in a given area using 240-grit sandpaper to make the first cut quickly. Our Plymouth's body shell has very few flat surfaces with compound curves galore, so a variety of blocks were used. We followed the contour of the sheetmetal, sanding at slight angles to establish a crisscross pattern.
In a few areas we discovered some minor filling would be required. Those areas were sanded with a dual-action sander and a thin coat of filler was applied. As D839 is to be applied over an adhesion primer any bare metal was spot primed with DP90LF before more primer surfacer was applied.
After the necessary repairs were made, the next step was wet sanding the body with 400-grit wet/dry sandpaper. We used a spray bottle to wet the surface and occasionally dipped the sanding block into a bucket of water to keep the sandpaper from clogging. The area being sanded was wiped off with a damp sponge to remove any residue that accumulated.
With no new faults found, the final step in getting the body ready for paint was one more round of wet sanding block sanding with blocks and 600-grit paper. At that point our Plymouth's body was as smooth as a cue ball and ready for paint for color. It also meant that we had the hood, doors, fenders, and a trunklid full of louvers to work on next and that means there are more block parties to come.