This filler will shrink slightly when it dries, so don’t be afraid to
leave it a little thick. You can also keep the stuff in the covered jar
for a while, so if you need to add more filler later, that is not a
problem.

After drying for at least a day, the filler will be the same color and
consistency as the original plastic part, and the seams can be
carved, sanded and shaped just as easily as the plastic parts (which
isn’t saying much, granted). Be careful sanding, or you’ll just melt
the plastic and make a mess. I use very coarse (60 grit) sanding
blocks to begin the shaping, and progress to finer grades as I go
along.

Eventually the parts will be ready for primer and paint.