Prepping For Show-Quality Paint
It has long been said that a paint job is only as good as the prep, but that leaves out the answer of just how to get there. A decent paint job will take a fairly intensive amount of prep work-undeniably hard and skilled work. Raising the bar and going for the ultimate glamour finish seen on a show-car takes exponentially more of the same. For the paint work on our project Corvette, we substantially disassembled the car, stripped off all traces of the old finish, and primers. We were fairly well invested in effort before even lifting a spray gun. We were well past the point of no return here, and there was no way we'd compromise the job now with a hurried prep job. No, the goal here is to make the car's body as straight and sharp as a pin, for a flawless look once the color is applied.
The Fill Is In
Our project car had never suffered any accident damage, and the fiberglass below was in exactly the condition it left GM-no better, no worse. The fact is that these cars weren't built as showpieces in terms of body prep, and virtually every one of them will exhibit minor flaws under heavy scrutiny. Our car was no exception, and the bare, stripped body panels told the tale. The relatively smooth No. 220-grit wet sanding given to the surface as the final stripping stage makes it fairly easy to pinpoint these surface flaws by feel. A minor dip here, a little rise there, we could discern the kinds of flaws that separate a stock body from custom show-quality bodywork. Some guys will cake on heavy layers of primer to deal with all but the most flagrant defects, but this isn't the best approach. The only real choice here is using polyester filler in a very light skim wherever we could feel a defect.
Working with filler, like most paint and body tasks, is an acquired skill. The filler coat must be skimmed on in just enough quantity to fill the flaw, without excessive buildup. It also helps to taper the application all around, avoiding laying down a hard edge that will be difficult to feather later when sanding. Working a fiberglass body takes more finesse than a steel car, where a rough cut with No. 36 grit or files can be applied without worrying about the adjacent surface. Fiberglass is easy to gouge, so the best approach is careful application of the filler to very close to the needed shape, keeping the heavy sanding with rough grits to a minimum.
All the filler on this car was worked with No. 80 grit on a long, rigid sanding board. While some suggest that the highly curved body of the C3 Corvette makes the flat long board useless, there is actually no area of the car that cannot be worked successfully with this tool, once the technique is mastered. The result will be superior to what can be achieved with flexible rubber hoses, foam pads, stirring sticks, and a multitude of other sanding aids.
The key to making the long board work is the angle, finding the right curve to the motion, as well as the required progression as the tool is moved along the panel. The filler will change to a lighter shade of color as the sandpaper "hits," making it easy to see the progress or any low spots. If there is a low spot that doesn't clean when everything adjacent is right, avoid the temptation to change the angle or pressure point to "chase" that particular spot; it needs a re-application of filler. A finished fill should feel perfectly smooth to the hand, show even sanding stroke marks over the entire surface, and have a smooth, thin, and nearly transparent feather edge into the adjacent area. Once the filler is shaped with No. 80 grit, a final sanding with No. 180 grit is done to reduce the depth of the sanding scratches, which will ease the amount of filling later with primer, and prevent the potential for visible sanding scratches after the paint and primer ages.
At one time, polyester filler was always glazed with spot putty to fill the rough grain and pinholes common to older fillers. The best of the modern fillers apply and sand so smoothly that a glaze coat is usually not necessary, especially if the filler is applied smoothly and not over-worked as it begins to gel. If required, a polyester-based spot putty can be applied to fill deeper sanding scratches and evident cratering or pinholes. If it is not necessary, it is best to avoid the glaze, since the repair will need to be reworked and shaped once again when sanding the glaze. We used Evercoat Rage Extreme filler and found it deserves its reputation as a premium product. Evercoat Metal Glaze was used where we needed it. Avoid older air-dry acrylic spot putties and glazes for anything but the most minute flaw, since these products are slow drying and have high shrinkage when applied in anything but the thinnest sections.
After we had the body worked with filler as perfect as we could get, it was time to prime. For this, we used Sherwin-Williams MP75 surfacer-an epoxy-based product that has excellent filling and sanding properties, and good adhesion to SMC, fiberglass, and metal. Preparation for the primer coats started with a thorough detail sanding, lightly skimming over the entire surface with No.180 dry and a hand pad. All the edges were detailed, eliminating filler buildup and overly sharp edges. A red Scotchbrite scuff pad was used in the hinge box area and other recessed areas, such as the lower body flanges that had been previously stripped and sanded. The detail sanding ensures a clean surface, free from debris or splatter from the filler process. Once detail sanded, the car is dusted down with compressed air to remove sanding dust and moved into the booth for taping.
Our preference for taping is to completely close off all the areas not to be sprayed. Not only does this reduce the tendency for overspray getting where you don't want it, but it also leaves only the parts to be painted exposed, sealing off areas such as the engine bay, cowl or chassis. This greatly reduces the possibility of debris blowing out of those areas and into the primer. We taped to the back of the body flanges all around the car's lower body, door jambs, and engine bay. Taping to the rear of the flanges gives complete paint coverage in the areas that are supposed to be painted without shoddy tape lines. Once taped, the panels are wiped down with a solvent wax and grease remover to clean any oils or contaminants that may have resulted from handling. Just before spraying, the car was air blown again and lightly wiped down with a tack rag. It pays to have a very clean surface, even for the primer coats.
Three even coats of MP75 were applied, allowing the solvents to flash between coats. The smoother the primer is applied, the more effective it will be in leveling the body surface later when it is block sanded, and less work will be required. We used one grade slower reducer in the primer, which improved the surface finish, but slowed the flash time. After the primer was dry, it was sprayed with a fog of black guidecoat, which is an important aid in sanding. We used black basecoat with extreme reduction, fogged on with high pressure to create our guidecoat, although commercial mixes of specialty guidecoat are available. Do not use a spray can enamel as a guidecoat; it is too coarse of a pattern and will tend to clog the sandpaper, especially if dry-sanding. Even in primer our Vette looked great, a big improvement from the patchwork of body filler we had been staring at. Once the primer is applied and cured, the block sanding process begins.
As with the preceding bodywork, block sanding is a job that requires learned skill. We once again primarily used hard, flat sanding boards for the block sanding, both the long board and a quarter-sheet short board. Our preference in block sanding is to start out with a fairly aggressive cut, using No. 180 dry paper. The coarser paper will level much better than a finer grade, which will tend to "float" over flaws rather than cut. The guidecoat should be evenly wiped away as the sanding progresses, showing a level panel. Dark spots where the guidecoat remains indicates a low area, meaning the panel needs more sanding to get it perfectly level. If the sanding starts breaking through the primer before the low spot indicated by the guidecoat cleans up, the flaw is too deep and will need additional attention. Here a judgment call is made whether to use a filling glaze or another primer application, depending on the extent of the defect.
After completing the block sanding with No. 180 grit, and any minor flaws are filled, we like to go in with a second application of surfacer. At this point, the surface should be nearly perfect and level, and the second application of primer is for texture, to fill the relatively rough sanding scratches. First, the car is detail sanded again, this time with No. 320 dry. Going over the surface with the No. 320-grit paper will reduce the sanding scratches that will need to be filled in the second primer session. Before the second primer application, we re-applied seam sealer as required at the hinge box and jamb seams, which had been removed when the car was stripped. The car is once again solvent cleaned, air dusted, and wiped with a tack rag. Because we were dry sanding, we did not remove the masking from the first primer session. Some painters prefer wet sanding, which requires removing the masking job and retaping. The second primer application is lighter than the first, since little filling is required. Two coats to give even coverage should be sufficient, followed by a fogging of guidecoat.
After the second primer application, the car is wet block sanded with No. 400-grit sandpaper. This final block sanding will provide additional leveling to the panels and a smooth silky surface for the finish painting to come. You know that it's perfect if the guidecoat washes away evenly in the final wet-sanding process, and the wet primer reflects a dead-level surface. This is the surface that great paintwork is built on and makes all the effort worthwhile.