On Sun, 17 Jul 2005 02:38:04 +0200, "Q" wrote:
Any clue if boatwax will work?
There are lots of things that will work, but you'd need to do more
experimenting with them. The special purpose waxes aren't much more
expensive than car wax, floor wax, etc. But for two parts, you could
skip the wax altogether if you use a secure layer of PVA.
PVA is regular (white) wood glue ?... I could add a little destilled water
and spray it on
PolyVinyl Alcohol. A liter of it should be plenty for your project.
If you can't find a local place to buy proper supplies, just take some
empty bottles and a box of donuts to a local fiberglass shop.
Hopefully one that's used to building and prepping their own molds.
Someone mentioned using babypowder on the mold as a release.. Will that
work?
I can't imagine how. I'm guessing that you'll be brushing gelcoat onto
your prepped mold. Whatever's under your brush strokes can't be
disturbed.
This might be a good time to mention that gelcoat isn't really
necessary. You could brush on a layer of resin instead, let dry, then
start laminating. You'd probably get a bit more print-through, and
you'd have to paint the finished horns.
I could integrate a quickrelease fitting for the airhose in the mold, so I
can blow air between the mold and the piece.
That could help a lot in the worst case, but shouldn't be necessary if
you do the prep right. If the horn is going to have a long throat like
the one in the photo, then put any extra time into making that area
nice. As much taper as you can, straight and shiny surface, heavy PVA.
I could do just a few layers on the mold, let it harden and add more layers
after I remove it from the mold... This would help the shrinking problem to
some degree, or am I way off track ?
That might help a little, but you could be surprised how much a part
that isn't fully cured will distort if it's unsupported and you add
thick laminations. So long as the mold has been tested to make sure it
will release cleanly, I'd finish laminating the part while still on
the mold, and then let it cure for at least a day before releasing.
Glenn's advice was very good, it's obvious he has some experience. I
only disagreed on a couple of minor points - vacuum bagging, not worth
the trouble IMO for two relatively small parts. Ditto for roving. I'd
get some heavy mat (random strands bonded with a resin-soluble
binder). You can split it into thinner layers if you like. You can wet
it in place, but if it were me I'd lay the mat onto a flat, clean and
non-absorbent surface like Formica, wet it out fully and quickly, and
then pick up the slab of gooey glass and apply to the mold. As the
binder dissolves, the mat can be stretched to fit contours. So your
minimum shopping list could be: PVA, resin (promoted), catalyst, mat,
acetone, brushes and rollers. BTW, getting polyester resin on your
skin isn't anything to worry about. If it was I'd be dead by now. :-)
Washing your hands with acetone is *not* a good idea, although it's
done all the time.
FWIW, my shop once made an assortment of horns (a few dozen of each
size) much like the one in your photo. The largest was about 4 feet
tall. We used heavy gelcoat, one layer of chopped glass strand (from a
big machine), fitted Divinicell foam bonded over the flat areas, and
then more chopped glass over the whole thing.
Wayne
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