I wouldn't worry about the seam (5x7 vs 4x8). Scarfing isn't hard, but this
would probably be the place to use the technique of just putting fiberglass
tape on each side of the plywood along the joint. Your structure makes up
for any lack of strength and you can fair in the lump easily.

I'd think CDX was the way to go. HDO is much nicer with fewer voids and
more plies, but you don't need it and it's pricier. For boats, it takes
bends with fewer lumps.

Too bad you didn't buy the lumber before Katrina hit.

Roger

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

wrote in message
oups.com...
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.