This step of priming and sanding the fiberglass surface is to properly prepare the area for the final step of painting and buffing. Preparing the fiberglass surface for paint is a process that will require absolute attention to detail. Attempts to shortcut the process with hasty repairs will only result in wasted time and money, in addition to an inferior repair. The ultimate goal is to produce a surface that is smooth and free of flaws.

At this stage of the fiberglass surface repair, flaws and holes should now be appropriately filled and built up to produce a level surface. (See Tech Tip - Fiberglass Repair.) Proceed now with a clean cloth and Diamont RM 900 Pre-Kleano to thoroughly clean the surface. Painted surfaces should be cleaned using a wax and grease remover.

Exterior temperatures determine primer dry times. Avoid performing repair work using primers while parked in direct sunlight.

Sandpaper is labeled in grit size - the higher the grit number, the finer the finish, the lower the grit number, the coarser the scratch. Starting with a too fine grit size will require increased time to complete the job. Using too coarse of grit in an attempt to hasten the project may instead create unnecessary scratching that will need to be removed. Insure that you are adequately equipped with the recommended sandpaper grit sizes prior to undertaking this stage of fiberglass repair.

Existing paint areas should be sanded using 240-grit sandpaper. Finish with 600-grit sandpaper to remove gloss. Once the site is sanded smooth, clean the area using wax and grease remover. Check for imperfections by rubbing a bare hand over the surface prior to the final cleaning. Oils present in the skin will affect the adhesion process, and can destroy the durability of the finish product. After checking the site for imperfections, once more thoroughly clean the surface.

After the area has been cleaned, protect adjacent areas from over spray. Use masking paper and 3M masking tape in an amount that will adequately guard against penetration or seepage.

When spraying primers, the proper NIOSH/MSHA respirator must be used. DP20 Diamond Prime is the recommended primer for use. This two-part catalyzed urethane primer will produce a hard durable surface for proper paint adhesion. Proper mixing of hardeners, primer and reducer is important. Measure chemicals carefully following product label instructions. Never rely on estimates. Generally, the mixing ratio is 4 parts primer-1 part hardener-1 part reducer. Two or three medium wet coats should be applied using 45 to 55 pounds of pressure on a conventional siphon spray gun, or 20 to 30 pounds going into a high volume low-pressure spray gun with 6 to 10 pounds at the cap. Allow 5 to 10 minutes flash time between each coat. Primer curing times will vary depending on reducer and hardener used in mixing. Times will also be affected by temperatures when force drying.

Resist the urge to rush this process. The DP20 should be sanded within 12 hours. In the event the primer is allowed to sit for longer than 12 hours, additional work may be required prior to painting. During all sanding, a particle dust mask and safety glasses should be worn.

After all coats of primer have been applied and allowed to properly cure, it is then time to apply a Guide Coat before sanding. A Guide Coat is a very light mist sprayed over the cured primer used to detect imperfections, low areas or scratches.

Once the area is primed and dried, additional sanding is required. The quality in the finish will mandate the number of times this sanding process should be repeated. In this stage, the finer the sandpaper, the better the finished product, but ratings no higher than P600-grit should be used to finish sanding the primer.

The sanding process used after the primer has dried typically consists of both wet and dry sanding. Dry sanding is recommended for best results. Use sanding blocks during this process to insure a level surface is maintained. All sanding in small areas should be performed by hand. An orbital sander can be used to sand larger repair areas, but should be handled with caution as the orbital sander used during this process may completely remove the primer and burn the finish.

Prepare the clear blend edge using a red or gray Scotch Brite pad. The clear blend edge should be 4 to 5 inches back from the primer area, and is normally prepared prior to the top coating process.

When the desired finished surface has been obtained, wipe the entire area with a pre-paint cleaner, or a final wipe. Blow off dust using 30-35 pounds of air pressure from an air nozzle. A tack rag can be used to remove dust particles.