Guys:

There may be some confusion here in the matter of what constitutes
"filling" and the role of gelcoats.

With the Rutan methods, the external cloth layer is raw after the
initial open layup and the entire surface requires filling of the cloth
weave and whatever it takes to correct minor errors in shape. That's a
lot of stuff and a lot of sanding, even if most of the light micro gets
removed.

If a mold is used, the only "filling" required is that needed to fill
minor blemishes, including small indentations used to register parts in
the molds. The total volume of added non-structural material required
here is tiny, so even paint primer may be sufficient in most cases.

Gelcoats are very widely used in the general composites industry,
particularly for high production molded polyester boats and car bodies.
Gelcoats can be any pretty color, can be sprayed into the mold by
relatively unskilled workers, and protect the mold surface from dust and
dead bugs. Gelcoats do not contain any reinforcing fibers, thus are not
composite materials, and do not contribute to the strength of the
structure - only weight. We do not use gelcoats for high performance
aircraft structures for reasons of weight and compatibility. I am a
little out of date with my sources, but the mainline industry does not
generally even offer true gelcoats appropiate for the epoxies we use.

My recollection is a little hazy, but I recall an exchange of
communication with Steve Rahm several years ago where he had had good
luck spraying the mold with regular Home Depot latex paint (Yeah!). I
preach the use of vacuum-formed molds where the actual mold surface is
bagging film. Release from dry bagging film is totally reliable, so the
builder gets a perfect release with no mold coating at all, thus no
added weight or compromise with a future painted surface. This
discussion with Steve arose because one of my sources reported that
Lotus Cars had used a process they called "transfer painting", where the
waxed mold was spray painted with polyurethane paint and cured. Lotus
then used a conventional thick gelcoat and a polyester layup -- but they
weren't going to fly the result. The source recommended the latex paint
trick and Steve tried it.

Zeke Smith