When building a street rod, few things are more disappointing than working hard and spending money on some aspect of its construction only to have the results fall far short of your expectations. In some cases minor flaws are only obvious to those who know their cars intimately and will never be noticed by anyone else, but then there are those that seem like they could be seen from space—and those are usually in the paintwork, or more accurately the surface below.

While it's obvious that laying down paint is more complicated than pointing the spray gun the general direction of the car and pulling the trigger, regardless of how well paint is applied it will never make bad bodywork better. Paint a corncob and it's still going to look rough, the reason is the paint will not level the surface but follow it. Those spectacular paintjobs we've all seen on the pages of Street Rodder and in person look that way because they are extremely reflective and a smooth surface is more reflective than a rough one. And block sanding is the secret to a flat surface.

Block sanding is a term that most of those in the hobby have heard, however it is still confused with hand sanding, which should be avoided. The problem with hand sanding is that pressure is unevenly applied to the sandpaper; look at a handprint on a flat surface to see what we're talking about. (Editor's note: The staff agrees with Ron as his hand print oddly resembles a paw.) We've said this before but fingers are for scratching, hands are for holding, and blocks are for sanding.

One of the common questions about sanding in general is what grit sandpaper to use. Coarse grits labeled in the 36-80 range are used for removing old paint, filler, and rust. Grits between 100 and 320 are used to smooth body filler and primer surfacer. To prepare primer for paint, 400- to 600-grit abrasives will smooth the surface while still giving the paint something to "bite" into. Finally, extremely fine grits, such as 1,000 and up, are for color sanding prior to buffing.

Along with grit, two additional terms that will be used to describe sandpaper are open coat and closed coat. Basically this describes how tightly packed the grit is on the backing material. Open coat abrasives have the grit spaced further apart, which helps reduce clogging when soft materials are being sanded. Closed coat means that the grit is packet tightly together and all of the backing is covered. Closed coat sandpaper is more aggressive simply because there is more grit on the backing.

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.

SOURCE
PPG Refinish
Strongsville
OH
440-572-2800
www.ppg.com/coatings/refinish
Jake's Place
541-997-9297
Eastwood Company
263 Shoemaker Road
Pottstown
PA  19464
800-343-9353
http://www.eastwood.com
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