Talk to any professional paint sprayer and they will inevitably tell you that a perfect topcoat can only be achieved if the preparation work on an auto body panel is perfect as well. Bodyworks garages use 2K primer on repaired vehicles, especially over areas that have received an application of body filler. Primer is used to create a built layer of material over repaired surfaces so it can be rubbed down to a smooth, flat finish. It also has noncorrosive properties that stop water from penetrating, which can prevent problems with rust in the future.
Apply a light application of polyester spot putty into any pinholes or deep scratches in filler work by adding the material to the narrow edge of a plastic spreader and wiping into the filler at a 45 degree angle. Leave the stopper to dry for 15 minutes, and sand down to a flat even surface with P320-grit abrasive paper. Use compressed air to blow off excess dust and mask the vehicle for priming.
Place the vehicle in an area with sufficient ventilation before priming begins. Activate the 2K primer at the recommended ratio with a compatible hardener and add 10 per cent of compatible reducer before stirring thoroughly to mix the components together. Add the 2K primer to a gravity-fed spray gun with a fluid tip set-up of 1.4 millimetres.
Apply one full coat of activated 2K primer over the repair from a distance of 6 inches, ensuring that the arm holding the spray gun continues to move at all times to avoid the risk of runs or sags. Overlap each horizontal movement by 30 to 40 per cent to ensure the edges of the 2K primer merge into each other and to avoid dry patches. Leave the first coat to cure for 15 minutes before adding a second and third coat. Remember to leave 15 minutes curing time after the second coat as well.
Use infrared lamps to dry the 2K primer for 30 minutes, or if lamps are unavailable, leave to dry overnight. Leave the masking paper in place to protect adjacent panels when rubbing down begins. Apply a light coat of matt black paint over the 2K primer once it has dried to produce a mottled guide coat that will highlight imperfections when the material is rubbed down.
Wrap a sheet of P800-grit wet-and-dry sandpaper around a rubber sanding block and dip into a bucket of clean, warm water to dampen. Lay the paper and block flat against the panel and rub down the 2K primer to reduce built layers until a flat, even surface has been achieved. Remember to visually check the guide coat for signs of pinholes or scratching and add more polyester spot putty as required.
Rub down the putty with P800-grit wet-and-dry paper once the putty has dried, and dry the panel with a piece of clean cloth. Remove old masking paper and clean the panel thoroughly with degreaser before masking again with new tape and paper so that the vehicle can be painted.
Always wear suitable respiratory equipment when spraying 2K primers.