Repair Or Replace?
"If there's rust in more than one or two small areas, replace the entire panel. Rust is like an iceberg. Whatever you see on the outside is only small portion of what's actually there. If you see a dime-size rust hole, there's going to be five times that amount of rust deeper in the panel. So if you're just cutting out one small area of rust and think you're getting all of it, you're fooling yourself. If you just patch one spot, chances are something else will pop up 2 inches away from it six months later. On a quality repair, you want to replace the whole panel and be done with it forever. If a dent in a panel can't be worked out within 11/48-11/416 inch of its original shape, replace it. A lot of people will use 11/42-1 inch of body filler, but filler isn't metal. Eventually you will have problems with it. You might as well spend the money up front rather than have to spend it again later."
Sanding
Prep work involves two different types of sanding: block-sanding and color-sanding. Block-sanding is performed prior to laying down paint and removes waves, scratches, pinholes, and other imperfections in the body. This prevents them from showing through to the paint. During color-sanding, the clearcoat (or the actual paint on a single-stage finish) is sanded down to remove orange peel, dust nibs, fine scratches, and runs in the paint. The finish is then polished to make it as slick as a piece of glass. "Color-sanding can also make paint that's several years old look new again," explains Rodney. "We usually put an extra coat or two of clear on car so that if the paint gets scratches or swirl marks there's enough material to color sand the car again."
Primer
Lots of hot rodders drive around in gray stuff as if it's paint, but primer is not a sealer. It's porous, contains lots of talc, and absorbs moisture. "Primer is like a sponge and holds five times its weight in moisture," says Rodney. With regular primers, moisture will go straight to the metal and start rusting from the inside. If you don't plan on painting your car for a while, then use an epoxy primer instead, which seals off the metal. With epoxy primers, even if it gets wet, the water won't be able to penetrate the metal. "Use the same paint line through the entire project. Don't mix and match different brands, because many times they're not compatible with one another. If you put a high-dollar paint on top of a cheap primer, it will break down and take the paint with it."
Panel Alignment
Properly aligning panels is one of hardest things to get right when building a car. Rodney suggests trying to set gaps at 5 mm or 11/44 inch at most. "GM used stacks of shims in older cars to align panels," says Rodney. "There are many different ways these panels can be adjusted, so it's really hard to figure out where to put shims to move them in the desired directions." If attempting to paint a car yourself, a reasonable alternative is having a shop line everything up for you for $100-$200. "Anytime the panels are removed after that, you can take shims out, mark where they go, and get everything to line up perfectly during reassembly."