You CAN build it out of just fiberglass, but, depending on the shape and
design details, this will be costlier and heavier than a composite. Glass
is flimsy, and needs a backer (or core) and/or lots of integral framing.

You say weight is a concern, but the weight of the tub will be small
compared with the weight of a full tub. There's probably no sense in
doubling the price, to save, say, 25% of the weight (a common trade-off in
boatbuilding).

Plywood comes in many varieties and qualities. Interior grades use a
non-waterproof glue. I never knowingly use interior for ANYTHING, inside or
out. Exterior grades use a waterproof glue. They are fine for most amateur
boatbuilding. Marine grades are like exterior, but are manufactured with
no voids, or small airspaces in the middle plies. Until you get into
serious boatbuilding, this is not a concern. Definitely not a concern with
a wood/glass hot tub - if the wood is consistently wet, you have bigger
problems. Marine grade also tends to be made up of higher-grade veneers
than lumberyard exterior.

I would suggest 3/4" exterior ply, (like CDX) screwed to 2x framing, 12" or
so O.C. Fair and fill the interior (round the corners, maybe 1" OR 2"
radius) then use the Vaitses technique http://tinyurl.com/8k9uj to sheath it
with glass. Epoxy is 'better' than polyester for fiberglassing, but more
expensive, and less familiar in the auto world.

Good luck!
Sal's Dad

-- 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