It's essential to prime a car before spraying on any surface coat, even if just touching up. The primer seals out rust and adheres to metal better than paint. In addition, it is chemically formulated so that paint adheres better to it than to bare metal. If you paint without it, you're likely to see chipping and flaking of a car's top coat. When you spray the primer, you don't have to be as careful as when you're spraying the topcoat, but do you have to make sure all areas are evenly covered.
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Things you need
- Masking tape
- Masking paper
- Wet-dry sandpaper
- Rust inhibitor
- Paint brush
- Body filler
- Orbital sander
- Air spray gun
- Sanding block
Mask off any areas around the area you plan to spray with masking tape and paper. Wrap tape around small surfaces like door handles and spread paper over larger ones. Be sure to tape individual sheets of paper together so they don't separate when the spray gun blows on them.
Sand down rusty areas with wet-dry sandpaper. Use a medium to coarse grade to scrape off the rust and flatten the metal. Spread rust inhibitor on all rusty areas with a small paintbrush and let it dry.
Mix fibreglass body filler with the hardener that comes with it and spread it over the rust with a plastic applicator or a putty knife. Scrape the edges so they feather into the metal, then let the hardener set for two to three hours. Sand down the body filler with an orbital sander and fine to very fine wet-dry sandpaper. When finished, the sanded area should match the contour of the surrounding metal.
Unscrew the cup of an air sprayer and fill it about 3/4 full with primer. Thin the primer, if necessary, with an appropriate reducer. Consult the label on the can of primer for the type of reducer to use and the amount to add. Screw the cup back onto the gun and plug the gun into an air compressor.
Spray all rusted and bare metal areas with a steady flowing motion, overlapping the spray pattern by about 50 per cent on each pass. Your goal is to leave a solid surface of primer with no voids. One coat is usually sufficient to do this.
Hold the gun further from the surface and spray more quickly if the primer runs or drips. It is not necessary to leave a heavy coat of material. You have covered the surface sufficiently if it is uniformly the same colour as the primer when you are done.
Pour unused material back into the can when you are done and fill the cup about half-full with reducer. Spray the reducer through the gun, then pour it out and wipe the cup clean with a rag. Unscrew the tip and clean it with a rag moistened with reducer.
Block-sand the primer when it dries with a sanding block and 400-grit wet-dry sandpaper to knock down high areas and create a smooth substrate for the topcoat.
Tips and warnings
- Spraying primer from a can is a cost-effective way to prepare smaller surfaces on your car for touch-up. If the primer drips or runs, wait for it to dry completely before you try to sand it down.
- Wear a respirator when spraying metal primer. The volatile solvents can damage your lungs and nervous system.
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