Check aircraft spruce for randolph sloshing compound or brushable
pro-seal. Both are fuel proof coatings used in aircraft tanks and
either one should fix your problem.

Rufus Laggren wrote:

1) Are you absolutely SURE you have located the source of the leak?

No, but 98%

You
said you saw and felt GRP fibers, but you didn't say you saw or felt
diesel.

Oh, I saw and felt the Diesel, all right. It goes into the bilge.

Is it possible a fitting somewhere leaks?

No. I pressure-tested the tank, fittings and hoses. The leak is in the tank;
all fittings are in the access plate on top.

2) If the layout has imperfections, the leak could travel several (or
more) inches inside the laminate before appearing. Fixing one spot on
the exterior would thus be problematic, especially if the diesel is
attacking the epoxy.

I can't get at the exterior; only the inside.

How big is the tank?

75 gallons

How old?

25 years

How is it built: Cored (eg. plywood)?

The tank is a centerline tank, vee-shaped, 26" deep and about 4' long. It was
made using 1-1/2 oz mat on refrigerator cardboard form, then 3 layers of 24 oz,
fabmat and 4 layers of 1-1/2 oz mat, and insulated with 1/2" urethane foam
before being lowered into the eggcrate. Inside, it was coated with a "special
chemical-resistant resin." I believe it was epoxy, but whatever it was, it
"responded to the catylist so quickly and hardened so much that it resisted all
sanding/grinding attempts."

Access ports?

One SS on top, approx 18" X 8"

Baffels?

Yes

How is it installed/supported?

The tank rests on glassed ply 2" above the fabmat which forms the top of the
keel ballast.

Was any protective coating applied inside the tank?

see above

Do you only have the one fuel tank, or
do you have a second separate tank that will serve if the repair gets
too interesting to complete quickly?

I have two others, each 25 gal. But I sailed her all last winter and just kept
wringing out a blige sock into a bucket every week. It isn't a big leak, just a
PITA, and I don't like Diesel in my bilge; dangerous and it stinks.

Given the access you have to the tank, can you do the GRP repair well
enough for this demanding application?

I'm sure I could if I choose the right materials.

If you're not sure, is it worth
the gamble or would it make more sense to open up the access some more,
even it it makes it harder to button it all up again?

Perhaps. I hadn't thought of that. But if I open it that much I may be able to
stuff a blivet in there.

Also, given the
access (or lack), how much of the surface of the tank can you see, and
is it likely there are other leaks?

I can see most of the inside (if I were there - she's in the Caribbean and I'm
in the States now) through the access port, except way down behind the baffles.
Since the leak stops when the fuel level gets low, I believe I can see where
there's a problem.

John