Bob wrote:

Anybody out there acually use a plywood/epoxy tank that has worked fine
for 10 years?
Bob


A tale of two ****ties:

About six years ago I boat my boat from a friend of mine (fellow
engineer) who had built his own plywood epoxy tank. Like yours it was
an odd shape and needed a custom job. He used standard 1/4" plywood and
covered both inside and out with one layer of glass. He used iron nails
to attach the lid to the box, sealing it with silicon window sealer.

I did not know this when I bought the boat, as the price was so cheap it
didn't warrant a survey. We moved on very shortly after that and
everything was fine with the holding tank for another six months. Then
we began accusing each other of farting.

I turns out that one layer of glass is micro porous. It took about 8
months for the sewage to seep through the inner layer, rot the plywood
core and then seep through the outer layer. It was not a pretty job to
remove this tank (which was 1/3 full at the time) and clean up the bilge.

I built a new tank from epoxy and glass mat using stiff cardboard as a
template. I used many layers (the final wall thickness was 3/8") and
added a powder to the epoxy (recommended by West Systems) specifically
for creating vapor/liquid barriers.

We have had no problems with it since, save for the inadequate job I did
venting the tank, but that was minor, providing we regularly add some
sort of sewage treatment.

Given all the options, this probably wasn't the cheapest or easiest, but
it did fix the problem.

Hope this helps.

Robb