On Sun, 17 Jul 2005 00:44:34 +0200, "Q" wrote:

I need to make a couple of horns ( for audio use ) and have a couple of
questions as I never worked with this stuff before...

The local Home Depot clone ( in scandinavia ) sells the glass fibres,
polyester resin, hardener and gelcoat wich should be everything I need ?

The mold is made from a big chunk of MDF ( Medium Density Fibreboard ) that
I laminated from several smaller sheets, then cut the horn flare contour in
it on a lathe..

To give you an idea of what I am trying to do I put a picture of an existing
horn he http://www.naeslund.dk/peter/files/oprydning/hpt42.jpg The mold
I made fits inside the horn.. Note that the actual horn I am trying to make
is round, not rectangular like this..

Can anyone give me some idiot proof instructions on where to go from here,
before I waste 100$ worth of fibreglassing ingredients?

If not, heres a few questions:

How do I get the surface thats touching the mold as smooth as possible? (
The mold was turned on a lathe, so it is smooth, but I'd prefer the fancy
shiny surface :-)

That surface needs to be as non-porous as possible. Seal it with
whatever your fiberglass supplier has on hand and recommends. Furane
is one product, sprays easily, dries fast, sands readily, polishes
shiny, smells terrible. It's not very durable though.

How do I keep the fibreglass from sticking to the mold?

After sealing the surface, use a specialty type paste wax. Apply wax,
let dry as recommended (perhaps overnight), and buff. Do about 6
coats. Do not try to speed the process. You might also bake the mold
in a warm oven between coats to help get the wax to penetrate the
surface. For at least the first part, apply a couple of coats of PVA
(from the fiberglass supplier). Spray it on if you can, it won't brush
very well on a waxed surface.

Any tips that may come in handy?

I can't tell you how many times I've heard that folks making their
first piece don't get the mold sealed and waxed properly, and end up
with the mold and the part permanently bonded together. So you might
try sealing and waxing some samples first to test that your technique
is adequate. The first release on a new mold can be troublesome. Try
getting a little compressed air into one corner. If that doesn't work,
try getting some water into the released areas to dissolve the PVA.
After the first release, add another coat of wax. If the first part
was tough to release, you might use PVA on the second one as well. But
if the mold is properly prepped, all releases after the first one
should be easy. The tail on your horn doesn't appear to have much
taper, and the part is going to shrink around the mold. So expect the
part to have a good grip on your mold in that area. You might make the
first part a single layer thick. That will give you some layup
practice, and enable you to peel the part off the mold if it's stuck
in some areas.

Wayne