First, and always TEST FIT ALL PARTS BEFORE PAINTING! It will only cost you time and frustration if you have to do a minor modification to a part that is already painted. In addition most parts suppliers are reluctant to accept a return if you find the part is not to spec and you have painted it.
Step 2. Preparation: Sand and Fill
Preparation Overview. The difference between a poor paint job and a terrific paint job often has little to do with spraying the paint. It has to do with preparation. This is not a difficult job, but it requires patience and an eye for perfection. And whether you are an amateur or professional, the techniques are the same. Basically, preparation is a three-step process which you repeat until the car is as smooth as you want it. First, you block sand to contour. Second, you fill low spots. Third, you primer. Each time you do this, you should be filling and block sanding smaller areas. If you goof and miss a high area or go too far on a low one, just fix it the next time around.
Door & Deck Lid Clearance. The first place you will need to begin looking for high and low spots is in the clearances around the doors and decklid. Your body and related parts should be aligned and shimmed in place before starting. Generally, you will want to make the gaps match width along the entire length of the opening, although ours are pretty good to begin with. (If the gaps seem way off, go back and recheck your alignment.)
Gap Width. Deciding on a correct width for the gaps is a compromise. Show cars, which are not driven on real roads, generally have narrower gaps than "drivers" because the motion of any car, metal or fiberglass, can cause narrower gaps to vibrate together and paint to chip. Keep in mind that the addition of primers and paints will reduce clearances. The gaps on our bodies are normally adequate, but still need to be checked to make sure there will be enough clearance with the addition of paint. You need to allow at least 1/16" clearance (before paint) in all body configurations, so check your clearance with the doors and decklid open as well as closed. Also check the clearance along the entire arc as you open and close them. You should make sure that there is at least 1/16" clearance at all times.
Adjusting the Gap. If you want to grind the gaps to make them wider or fill them in to make them narrower this is your next step. Always SAND or GRIND THOROUGHLY any area to be filled to assure adhesion of the filler. In areas that need more clearance, grind off high spots carefully. Don't go through the fiberglass layup!!! It is not a problem to grind through the gelcoat, as you can later cover the area with primer or filler. Remember to keep checking the contour of your gaps as you sand and fill.
Choose Your Putty. Use a good grade of catalyzed polyester body filler. (Be sure to use the catalyst recommended by the manufacturer - they are not all the same). Polyester fillers and primers have basically the same chemical composition as fiberglass resin so they should be totally compatible. In addition, catalyzed fillers and primers do not shrink like air-dry materials. Also, always use fillers that can be water-sanded. (Check the manufacturer's recommendation). These are easier to work with and do not absorb moisture, which can be a problem. Follow the manufacturer's directions for your particular putty, and fill the areas you have prepared.
Disassemble the Car. Now disassemble the car as much as is necessary for paint. If you are removing the body from the chassis, be careful to mark all shims, heights, and locations. You may want to do this in several steps: Remove the body while the doors and decklid are still on it. Next paint the chassis, body floor, and firewall. Now reassemble and re-shim the body before you remove the doors and decklid to paint them and the body.
Sand & Fill Seams. Before moving on to primering, you will want to do a little more sanding and filling. Sand all mold seams and other rough areas (including any filler you have used on the gaps) with a hand held block. This can be done with 80 to 220 grit paper; sand carefully to avoid doing any damage to areas surrounding mold seams. It is easy to over-sand and end up with a flat spot.
Sand & Fill Low/High Areas. Most professionals and other picky people would sand the entire body and related parts at this time, carefully watching for low and high spots. Low spots are easy to see because as you sand through the black gelcoat finish, it appears gray. The finish on low spots does not abrade if you are using a fairly stiff block, so these spots will remain black. (If you are sanding on other gelcoats that do not appear different colors, and also in subsequent steps on our parts, you will have to apply a guide coat to get this same effect.
High Spots. High spots will, of course, be the first areas to sand down. You may end up sanding all the way through the gelcoat and into the fiberglass layup. This is perfectly O.K. as long as you don't sand through the layup, because the area will be painted with primer. However, since our parts are very straight to begin with, this should be rare, so double-check! (This is a little different than the process you would use on metal. When you are block sanding metal and a high spot shows it cannot be sanded down, but must be worked down carefully with a hammer and dolly).
Sanding Blocks. Sanding blocks can be of any shape, size, or material to do the job you intend to do, but the more rigid the block, the better it will cut high spots and leave low spots. The more flexible it is (such as a rubber block) the easier it will follow contours (including ripples). Basically, you will want to start out with a harder block for your first sanding steps, and move to a softer block for your final sanding(s) with the finer grit paper.
Sanding Tips. When block sanding with any block, you must take care to use a diagonal or somewhat sideways operating pattern with the block to avoid cutting grooves with the sides of the sandpaper. Hold the block at an angle to the direction that you are sanding. When sanding into corners, where you are forced to hold the block head-on, care must be taken not to gouge material around the area to be sanded. It sometimes helps to make your own miniature sanding block by wrapping sand paper around paint stirsticks or other handy objects--but you still have to be careful.
Final Sanding & Filling. After this initial sanding, any areas that still appear black are low and should be filled. But before you fill, be sure to SAND or GRIND these areas to assure good bonding. Since you will be sanding off the black finish, you may want to mark the areas first. Once again, use a good grade of catalyzed polyester filler.