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

Hi,

Thanks for the replies. What grade of fiberglass would each of you
recommend (with plywood, or without plywood)? I was planning on buying
fiberglass cloth on eBay, and I've seen anywhere from 2oz to 45oz. The
shipping is usually more than the bid price.

Terry,

When you say "Took about 20 thin coats brushed on.", do you mean 20
finish coats of resin? How many layers of actual fiberglass cloth do
you recommend? Is it better to use a lighter grade glass, and use many
coats, than to use 1 large 45oz layer?

Thanks for the input.

wrote:

Hi,

Thanks for the replies. What grade of fiberglass would each of you
recommend (with plywood, or without plywood)? I was planning on buying
fiberglass cloth on eBay, and I've seen anywhere from 2oz to 45oz. The
shipping is usually more than the bid price.

Terry,

When you say "Took about 20 thin coats brushed on.", do you mean 20
finish coats of resin? How many layers of actual fiberglass cloth do
you recommend? Is it better to use a lighter grade glass, and use many
coats, than to use 1 large 45oz layer?

Thanks for the input.

My firt 8' dinghy job was formed over a saggy old wooden skiff. A
coat of wax paper, cloth, 2 or 3 layers of mat alternated with light
cloth, all done while green except for a break in the middle cured
overnight, rough sanded and washed with acetone in the morning,
followed by many coats of clear resin with like, 7 or 8 drops of
yellow pigment per mix. Just kept on mixing a coat's worth in a cup,
colour first, MEK next, paint it on, mix up, again, again,,, got the
speed mixed right to keep it all tacky, flowed on with an old brush
stiff and yucchy, squeegeelike, built up to about 1/16 thick self
levelled gelcoat, in an open garage in summer humidity. Filletted
inside corners with bog and a layer of cloth. Strong as a cast iron
bathtub, stiff, with a little hogging between the centre and each
end, hardly noticable. Planed at about 15 kt with me and a Suzuki
3.5 o/b on the flat. I steered "with my chin", hung over the front,
leaning to steer. Great fun. Traded it away, wish I hadn't, now.

I don't think it matters much what weight cloth, it was all ad hoc,
I had some scrap and bought some at the local race track f/g works.
I watched them make apple barrels and learned to keep an airtight
ammo box acetone bath to clean tools, especially a washer roller I
made up from threaded rod and loose alternated 2 sizes washers with
locked nuts to squeeze out air between glass lams.

The boat was about 1/4" thick or less and included a luan floor to
hold down 1.5" styrofoam, 3 plywood seats sitting on wooden 2x2
clamps, screwed from the outside. The gunnels were old plywood
strips, one in, one outside, screwed together and capped with a
layer of glass. Shoulda been 2 or 3 layers. Coulda been nice
mahogany, routed to fit over the rough edge. It got tore up dragging
it over stones. I whittled knees for the top of the transom and flat
stem, leaving holes for rope, forming handles screwed from the
outside, and painted all inside with leftover beige latex. A plywood
motor mount doubler for the transom incorperated into the stern
thwart. Space under the seats for my "golf club" bilge pump clamped
onto stiff plasic pipe 4 feet long, using a bag phone battery and
flex hose discharge. Putting the discharge under the water outside
the boat formed a syphon that meant the pump needed not work to
actually lift the water, it only worked to move the water sideways,
out of the boat. I used to go around the marina and pump out dingies
after a rain until the battery went flat, just for the hell of it.

They never found out it was me.

Couple of hundred bucks, then, altogether.

Fun.

If you cut holes big enough to pass curved through hull "nozzels"
with hosed clamped on and 5200 permanently glued to the inside, you
could route hoses to a "T" manifold assembly and valves, with pump
controller, or use the big nuts and clamp hoses on later if you have
ample room behind. Make sure hair, etc can't get sucked into a
recirculating drain and drown someone. Keep the pump intakes high on
the sides, and grated to protect your biggest pekker too, or use a
passive, nonsuction overflow collector, maybe perforated drain pipe,
possibly under a decorative, removeable side deck grating. Add a tub
/ shower surround kit, maybe some mirror tiles, and Bob is your
uncle. A timer on the pump would ensure eventual release of anyone
snared too far from the power switch. People have been drowned, dare
I say even worse, by recirculating spas.

I never heard of osmosis blister in fresh water, especially if the
tub is not always full.

Terry K