Kinda like to help here an speed the sanding an finishing up.This sorta falls into what i used to do.Grew up in a body shop did every thing from totals to custom dragsters,airbrush work to sandblasting windows designs.
The filler your usen gets very hard does an make a waterproof shell (great from keeping the chemicals in paint from eating up the foam ),i preferd the usc verson of that http://www.levineautoparts.com/duraglass.html
but their close .
Now havieng done some scuptures with this stuff steping up to a smoth surface just useing that filler alone is very very much a pita.You get pin holes an it top coats some itmes depending on humidity gumy( always suposed to keep old sand paper to break the gel gummness ) .So it makes for exclent ruff'n in but finish levling work, not so much.
We used to like to step up with http://www.levineautoparts.com/allmetal.html
still makeing a decent job water wise ,leveled well , and sands a bit better than the fiberglass .Some times has a gellish coat to it .Some times we'd get old cans ( main reson we quit usen it actualy )but once threw it sanded exlent an would not shrink.
But for this i'd go with http://yhst-13811118617756.stores.yahoo.net/usc-21000.html
easyer to work with by far. Makes good corners.Sands great 220 grit paper to this is like takeing 80gt to reg filler .and tapers out well with 400grit.
Toping out easy to use (dont make corners from this tho ) is http://yhst-13811118617756.stores.yahoo.net/usc-26006.html
.Not sure if your planing on wet sanding an priming a good epoxy primer an you got a slick finish .Guessing your gona go semi gloss on it.Which would give ya some room for error.It wold hide flaws a litle bit.
Good to see ya usen a good mask .Dust masks the thin ones just dont do enuff.Tween that an paint fumes ones lungs can get trashed over time .Did mine at least :/
One more thng as a melty ruff look you could brush fiberglass resen on the foam to begin with ,if its not thinded with laquer thnner it should not eat up the foam .Stoping their i dont think it would look bad with original source style imho.