^i was just going to suggest that.
i prefer either "easy sand" bondo or shortstrand fiberglass.
bondo if it's something small,no more than 1/8th of an inch to maximum 3/16th's of an inch. any thicker than that and you risk having it crack on you.
for the bigger holes/gaps,shortstrand fiberglass is where it's at.
with the bondo though,if you mix it correctly with the hardener,catch it at the right time and you can almost "grade" it like cheese.
wait too long and it'll be like sanding concrete.
i find that if you also use just a smidge less of hardener than recommended,it'll give you that much more time to mess with it.
keep in mind that with the shortstrand fiber,you won't be able to do the "cheese grader" technique,as it contains actual fiberglass strings in it.
above all else......DO NOT USE THE "RED" WANNA-BE PUTTY THAT'S IN A SQUEEZE TUBE.THAT STUFF NEVER COMPLETELY DRIES AND 99% OF THE TIME WILL JUST CRACK AND FLAKE RIGHT OFF!!!!!!
if it's only a one part putty,it won't be near as strong or resistant as a two part
.