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If the car you’re painting is heavily dented, the first thing you’ll need to do is to remove as many dents as you can to smooth out your painting surface. Glue an appropriately-sized suction cup to the center of the dent using a hot glue gun and wait 1-2 minutes for the glue to harden. Then, pull firmly yet gently on the retractable tab until the metal returns to its original shape.  You can pick up a dent repair kit from any auto supply store for just a few dollars. In certain places, like the hood, trunk, and rear panels, it may be possible to pound out dents from the inside of the body using a hammer and dolly. Mix your filler material to a cream consistency on a mixing board or piece of scrap cardboard. Then, spread it over any irregular areas in the body of the car using a clean spreader or buffing pad. It will help fill in these areas, resulting in a smooth, level painting surface.  Make sure you use enough putty to fill in each damaged spot completely. To deal with smaller imperfections like pinholes, try a glazing putty. Glaze putties are thinner than ordinary fillers, which allows them to penetrate into places that thicker products can’t. This will typically take 10-30 minutes, depending on the product you’re working with and the amount you’ve applied. In the meantime, avoid handling the putty. Otherwise, you may accidentally smudge an area you just repaired.  Body filler materials don’t technically “dry,” they cure via chemical reaction. For this reason, a heat lamp or hair dryer can come in handy to help speed things along.  Some body filler materials double as primers, which means you can paint right over them as soon as they’ve cured. Be sure to read the specifications on the product you’re using—it could end up saving you a lot of time. Once your putty has had time to cure completely, go over it with a sheet of sandpaper 150-180-grit sandpaper. Be sure to sand in all different directions, using vertical, lateral, and circular strokes to produce a seamless finish.  While sanding your filler material, it’s especially important to feather the edges to minimize the chances of lines or ridges showing up in the finished paint. A block sander can make smoothing large areas easier and more comfortable.
Pull out large dents using a dent repair kit. Fill in gouges, dings, and depressions with body filler putty. Wait for your filler material to harden. Sand the edges around the hardened filler to smooth it out.