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Once your expanding foam has dried, it will have expanded significantly and may extend a couple of inches away from the door. Using your utility knife, cut the excess foam away until the foam is slightly lower than the surface of the door. The expanding foam needs to be completely dry before you start trimming. If the center of the foam is still wet, wait a few hours until it's fully dry. Scoop or squeeze an auto-body filler out of its container and into a disposable bowl or any sort of plastic tray. Then squeeze in half as much hardener catalyst. This material will quickly harden and cover the outside of the door hole. These are both thick, putty-like liquids; use a popsicle stick to stir them until the substances are fully blended. Start out with about 1 teaspoon (4.9ml) of the auto-body filler and half as much catalyst. Mix more as needed. Both auto-body filler (such as Bondo) and a hardener catalyst should be available at a large hardware store. If not, check a home-supply store or an auto-body business. Use your popsicle stick to scoop up some of the filler mix and smear it over the dried foam that's filling the hole in your door. This material will dry and harden quickly, so you'll need to work fast. After about 5 minutes, once the filler has started to thicken, take the edge of a putty knife and scrape it across the filler. This will smooth the surface and remove any excess, semi-dry filler. Wait to proceed until the filler has fully dried. Use the sandpaper to sand the fully dried filler until it's completely smooth and flush with the surface of the door. Also sand off any leftover expanding foam or filler that has stuck to the plywood door material around the edges of the hold you've filled. Sandpaper of all grit consistencies will be available at your local hardware store.

Summary:
Cut the protruding foam. Mix auto-body filler and a hardener catalyst at a 2:1 ratio. Apply the filler mixture over the dried expanding foam. Sand the filler with 100-grit sandpaper.