Step 21: Sand the interior of your newly fiberglassed beaver
Let the fiberglass dry and air out for at least 24 hours before you start working on it.
Use a dremel with a sanding attachment to get rid of any sharp or misshapen areas. Sand by hand with sandpaper to get it as smooth as you want it. You can mix up resin to fill areas that need it as well, and coating it with some resin after you've smoothed out the surface will leave a nice finish.