Re: fibreglass molds

[Frank Pittelli <frank-at-rctankcombat.com>]

chris barthelson wrote:
> Hi all,
> I'm about to start work on my turret and I need to know a couple of 
> things. First, is the foam used to make a mold for fibreglass normal 
> floral foam is it it special in some way???. And do I have to coat it 
> with anything before glassing??? If so what is the cheapest way to do 
> this???.

The floral foam would probably suffice, although I'm not sure how well 
it sands to shape.  I've used the blue foam that they use for rigid 
insulation, which is fairly dense and sands quite nicely.

After you've carved and sanded it, you will want to seal it with 
polyurethane, epoxy or a slurry of white glue.  Basically, anything that 
will harden the surface and start to fill in the holes.  After the 
sealer dries, the surface will be rough and should be rough sanded again.

After sealing it, spread on a thin layer of wood filler (a thin slurry 
works best) and sand it smooth.  Don't press down too hard or you may 
deform the foam underneath.  After sanding the first coat, I usually 
lightly spray paint with a quick drying primer so that I can see all of 
the bumps and valleys.  Then, spread another layer of wood filler and 
repeat.

Don't worry too much about making the plug surface perfect, you'll have 
to sand the final casting anyway, when you can clean up any bumps or holes.

Remember, molding is all about surface preparation ... there is no 
substitution for patience.  It takes 3 or 4 filling and sanding passes 
to yield a nice surface, so get yourself a good stack of sandpaper, 
change the paper frequently and make long smooth strokes.

        Frank P.