Two things:
(1) coating 'clean'
aluminum . . . . to have a solid bond to aluminum you pretty much need to degrease, abrade and then 'prime' with one of the aluminum specific products ( like the aluminprep & alodine combo). This is because 'clean' aluminum usually has both some sort of greasy or wax from the foundry, and also oxidization, and leaving either will cause bonding problems. This is an issue with pretty much any 'thin' coating, less of an issue with 'thick' (the 5200 thickness and thicker) coatings that can remain intact with bubbles and poor bonding underneath. Note: it is possible to acid etch rather than abrade but is trickier, especially if you can not closely inspect it.
I have a lot of experience with aluminum coating.
(2) Making relatively thin
fiberglass layers actually totally fluid impermeable is more difficult than you might think - the glass weave is obviously not impermeable so you are depending on the resin having no pin-holes and developing no cracks, when the aluminum will be expanding and contracting with thermal changes. It can be done . . . but is trickier than you might expect and (usually) requires more layers of glass. The guys in the C-Class learned this about their carbon wing skins and superthin carbon laminates (the
core would soak pretty quickly because it was often not entirely vapor tight).
Really . . . if you are going to this effort . . . get a plastic tank and be done with it.