I wondered about porosity...but not with aluminum, my prior experience was with a cast iron master cylinder.
How about some sort of high-build primer, followed by a few coats of paint? If you can use a two-part epoxy primer/paint combo it will "cure" from the catalytic reaction and should hold up against the brake fluid, which might soften and disolve a regular paint.
Maybe a few heavy coats of powder coating?
Tough issue, Bill. I ended up replacing mine in the end...but, it needed to be replaced anyway to get better braking. You may find it easier to work on off the car...this has been a tough build!
Cheers!
Dugly
