You know, a lot of today's doors look like they're solid. But in fact, they're hollow, and you can end up with damage that looks like this. But don't throw that door away. Here's a repair that you can make in less than an hour.
With a utility knife, cut a bevel around the edge of the hole. Next, insert a piece of crumpled paper into the opening. Then fill the hole with aerosol foam insulation. Use the low expansion type and press it in firmly. When the insulation is dry, trim off the excess with a sharp knife.
Now we're gonna fill up the rest of this hole with something called polyester resin, otherwise known as auto body filler. It's two parts: the resin itself and catalyst. You’re simply gonna mix these two together. Mix the resin thoroughly and apply it with a putty knife. As soon as the resin starts to get rubbery, trim off the excess. Next, sand the surface smooth, using 100 grit paper. Then apply a coat of spackle or joint compound and smooth it off, using a putty knife that's wider than the patch.
When the spackle is dry, sand the surface one final time and brush on a coat of paint. Well, you'd never know there was a hole there, would you?