Kenneth wrote:

422 Barrier Coat Additive

A proprietary blend designed to improve cured epoxy's
moisture-exclusion effectiveness. 422 is used as a barrier
coating additive to help prevent gelcoat blistering. 422
also increases the epoxy's abrasion resistance. Cures to a
light gray color. Add to mixed resin/hardener at the rate of
15 to 20% by weight-3 tablespoons per 8 fl. oz. epoxy
(approximately 32 oz. per B group).

Aluminum powder (422), as described above, is used as an additive
specifically to provide a barrier coat (About 5 mils per coat max)
over a pock marked (blistered) polyester surface on a fiberglass boat.

It is strictly a repair process.

Definitely not an adhesive application.

But, it is my understanding that it is not completely
impervious to water damage either. The fact that West
Systems sells an additive to improve the moisture-exclusion
effectiveness of its own (highly regarded) epoxy would seem
to support my understanding.

Might I be wrong? Of course...

It is a matter of molecule size.

There are only about 4-5 manufacturers of base resin (part A)in the world.

Ciba, Dow & Shell come to mind.

There are litterly thousands of formulators such as Gougeon (West)who
provide epoxy systems for final application.

Basic epoxy for wood working applications can be considered waterproof.

You can improve the "waterproofness" by reducing the permeability
(reject smaller molecules) of that surface by applying epoxy coatings
specifically engineered for that task.

It is strictly a matter of chemistry.

The fact remains that general purpose epoxy is basically waterproof.


Lew