What do I need to fill a gash in front bumper cover?

I have a '06 GMC Envoy with a small gash in the front bumper cover. I purchased epoxy primer, half a pint of paint matching the paint code, and urethane clear coat. I know how to paint since I worked with a painter at a body shop for 2 years when I was younger. But we mostly always changed out bumper covers instead of repairing them. The gash does not go all the way through the bumper cover, just gashes into it. It's only about 3 inches long. I have a sanding block with 60, 120, 180, and 1000 grit sandpaper. Also, is regular solvent wax and grease remover ok to use on plastic bumpers before painting?