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

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

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,

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 absolutely use vinylester or epoxy resins for a hot
tub otherwise you are likely to get a very fast case of
osmotic blisters. You will need some sort of framework for
the glasswork, be it foam or wood.

Also make sure the final 2 inside layers are chopped strand
mat with a heavy coat of resin for waterproofness.

You can use a mix of alternating layers of CSM & woven
roving 18-24 oz

Evan Gatehouse

If you want to be creative with the shape of your hot tub I would suggest
you use the foam materials, Diab and Core-Cell are two good options. All
foam must be covered with glass and should tapered off where your jets and
drains are. Anywhere you are going to perforate the tub should have no
exposed foam. The vinyl ester resin subjected is a good option and I think
needed to avoid blisters on the finish surface. However it is a very
expensive product, to keep the cost down do all your laminations with a
general grade poly resin and the two surface layers of mat with the vinyl
product. (I would go three mat here). You should take a look at a few of the
sites that are out there on manufacturing with glass, there are a number of
way to make a break-away mould at a very low cost to produce a top of the
line product.

Good Luck
Doug Christie
wrote in message
ups.com...
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 again to all for the help. I plan on going to the local
university library to learn a bit more about fiberglass before making
my decision. Do you have any personal favorite books on the subject?
The Vaitses book has already been mentioned, it is on my list.

I'd like something fairly basic, nothing that gets too in-depth about
the chemistry, or about industrial-type methods. I'd like to learn
about covering plywood with glass, making a 1-use mold, and also
free-forming (doesn't have to all be the same book). Also I'd like to
learn about the different properties of mats/cloths, different types of
resins, gel coats, foams, etc. Thanks.

I don't think a university library will be much use . The sorts of things
you need to know are exactly what Universities are NOT about.

I'd spend a bit more time on the web, looking at manufacturer's sites. I'd
also do some careful cost estimates based on what you might end up paying
for appropriate materials. Then double it for the inevitable learning curve
and wastage costs for the first time one- off, add another 100% for fixing
it sometime in the future ... and compare this with what a commercial
product is worth. You might decide it's not something you want to do.
Perhaps talk to a small commercial manufacturer, see if you can make a mould
and have them produce from it ...
David
wrote in message
oups.com...
Hi,

Thanks again to all for the help. I plan on going to the local
university library to learn a bit more about fiberglass before making
my decision. Do you have any personal favorite books on the subject?
The Vaitses book has already been mentioned, it is on my list.

I'd like something fairly basic, nothing that gets too in-depth about
the chemistry, or about industrial-type methods. I'd like to learn
about covering plywood with glass, making a 1-use mold, and also
free-forming (doesn't have to all be the same book). Also I'd like to
learn about the different properties of mats/cloths, different types of
resins, gel coats, foams, etc. Thanks.

Try to find "The Gougeon Brothers on Boatbuilding." These are the guys that
developed and marketed West System epoxies and their newsletter has
described uses including lots of applications in swimming pools,
architectural stuff, etc. As you might suspect, the book describes the many
boats they've built using all sorts of techniques. Well written with lots
of pictures. Their techniques work well with System Three products, but
don't tell them. http://www.westsystem.com/

System Three gives away "The Epoxy Book" on their web site.
http://www.systemthree.com/index_2.asp
Free if you register and download and print it yourself. It covers much of
the same material in a lot fewer pages, but do read both. (Skip the first
few paragraphs of The Epoxy Book if you really don't want to hear about the
chemistry.)

Roger (I think S3 might have less blush than West, but then maybe I've
just been lucky.)

http://home.earthlink.net/~derbyrm

wrote in message
oups.com...
Hi,

Thanks again to all for the help. I plan on going to the local
university library to learn a bit more about fiberglass before making
my decision. Do you have any personal favorite books on the subject?
The Vaitses book has already been mentioned, it is on my list.

I'd like something fairly basic, nothing that gets too in-depth about
the chemistry, or about industrial-type methods. I'd like to learn
about covering plywood with glass, making a 1-use mold, and also
free-forming (doesn't have to all be the same book). Also I'd like to
learn about the different properties of mats/cloths, different types of
resins, gel coats, foams, etc. Thanks.

Hi,

Having read some of the references you folks directed me to, I think
I've eliminated some options. I'm going to either use:

- exterior grade (ie CDX) or high density overlay (HDO) plywood, with a
coat or two of quality epoxy, and make a 5'x7'x3' box.

I don't think I'd even glass it, just caulk the seams and put a liner
in. One problem with this method is that if I used 4x8 sheets of
plywood, the 5'x7' bottom would have to have a horizontal seam. I'm
guessing I'd spend $150-250, plus the liner.

- cold-mold a tub with more comfortable contours and curves, using 3-5
layers of approx 1/8" fir (because I have an enormous free supply)

I've read in an article (not sure of the date) that cold-molding cost
about $3.30 per pound, at 1 pound per square foot, including the cost
of the wood. My tub would be about 107 sq ft. So by that person's
calculation, it would cost just over $350 for the raw materials. I'm
wondering how much of that cost was due to the wood, and how much for
the epoxy. I have a good amount of free time, and would love an excuse
to learn this method, so labor isn't an issue.

- I've heard that there was an article in Home Mechanix (somewhere
around 1990, I haven't found it yet) that described plans for building
a tub by fiberglassing over masonite (hardboard). Sounds interesting,
but I'd like to find the article first. Masonite is very cheap, and
that "supply of fir" I have includes 2x4's, which could be used for
additional framing.

Thanks for the continuing help.