I suspect what your experiencing on the aluminum is
corrosion. Similar to rust. When it’s aloud to continue
over a long period of time it gets deep into the metal.
Aluminum brighteners only work on the surface and
as they are an acid can turn the aluminum black if
left on too long. Try some 0000 steel wool dry. Tear
off a few pieces switching to a fresh one from time to
time and keep scrubbing till there is no more
improvement. See what you think. The aluminum will
still be pitted and it’s will still show. If you can live with
that give it a polishing with an aluminum polish to
protect what you have done. Do a small part at first till
you get a feel for it. The only way to eliminate it is to
sand past it and the impurities in the early cast aluminum
make that pointless. Most people don’t feel its worth
the effort and if you find out you feel the same it just
means your normal.
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.