Question - how well will expoxy impregnated fiberglass mat adhere to a
well prepaired rough surface ?

Epoxy does not adhere to oxidized aluminum, which is to say, any aluminum
exposed to air, even for seconds. So you cannot just sandblast it and
expect it to bond.

I have heard of a technique where you first apply a thin epoxy layer to the
metal, and then immediately brush with a stainless brush through this
layer, which exposes fresh elemental aluminum but protects it from the air.
This is reputed to work but I have no experience with it myself. And I
don't know how well it holds up when immersed.