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.
 Polyester fillers like the...  Polyester fillers like the Evercoat products we used are the best way to work the panels to perfection. These newer formulas go on smoother and sand finer than the products of a generation ago. |  The seam line is always a...  The seam line is always a source of flaws and waviness. We skimmed it with a thin coat of filler to blend it smooth. Note that the application is even and relatively level. |  Virtually all our filler work...  Virtually all our filler work was cut with a long, hard sanding board and No. 80 grit paper. The long board takes quite a bit of technique on the curved surfaces of a C3. |
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.
This is what the filler should...
This is what the filler should look like when done-nearly translucent, feathered edges, and consistent sanding marks throughout. The surface should feel seamless when running a hand across it.
Prime Time
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.
A small impact crater in the...
A small impact crater in the jell-coat just above the left fender vent was the only damage to our mint '76 Corvette body. A smear of filler will patch this up effectively.
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.