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wrote:
Hi,

I know this is a bit off-topic for this group, but I'm not sure where
else to ask it. I'm going to build a hot tub (more like a soaking tub),
and I'd like to use fiberglass. I will first build a 2x4 frame, then
(maybe) attach plywood to the inner part of the frame, then apply
fiberglass cloth. But I'm trying to decide whether the plywood is
needed for structural reasons, or just to give shape to the fiberglass.

Do I need to use something heavy-duty (ie 3/4" ply)? Or can I just use
1/4" ply (or even just a stretched-out bed sheet) and fiberglass over
it? In the DIY car audio world, people sometimes use t-shirts to create
a sort of mold. If the fiberglass can bear the weight of the water, I'd
just assume not use any plywood. I fear that the heat and steam coming
off the hot tub might cause delamination with regular plywood (and
marine grade plywood is just too expensive for this project).

I'm open-minded to completely alternate building materials, but weight
is a concern, so no concrete. Due to the location this is being
installed, most store-bought tubs won't fit, so I can't simply buy a
used tub. Thanks for the help, and sorry if this is a bit off-topic.

Shamus

I would consider using chicken wire to form seat shapes, etc, and
almost anything, even newspaper, plastic sheet, to help retain the
juice of the first lay up layer. Fiberglass is plenty strong at 1/8
inch or more thickness, and boiling water won't hurt it. You will
want to lay up several layers, then fill, smooth, and paint with
more coloured resin. I did a dinghy that way, and it finished up
looking like professional gel coat. Took about 20 thin coats
brushed on. Self levelling paint, it dries glossy.

Lightly coarse sand between layers, and wash with acetone, if the
last layer isn't tacky anymore. Use lots of disposable gloves.

Use a wheeled roller to press out air bubbles. Wash the tool in
acetone as soon as you stop using it. You might want to get an ammo
box, airtight, for an acetone tank. Use a barrier coat cream for
your hands, arms, face. Ventilate well. Buy a cheap disposable
painter's coverall. Polyglass will not stick to polyethelene.

Don't mould yourself into a corner you can't get out of. You will
need to access the backside to plumb it, possibly a lift off topside
decking, even if you only need to thread hoses and ends onto glue
pads. I would do it in place, else it might not go up the stairs.

Terry K