wrote:
Would you ever use sandpaper to prepare Aluminum for
epoxy, or rely entirely on chemical pre-treatment?

3M Scotchbrite pads nicely scuff up the surface for bonding or
prime/paint. Maroon is finer than green, either works fine (just don't
go nuts with the green since it is coarser). You can find the green
just about anywhere, but I've only found the maroon (7447) in a few
stores or online.

Also, since you're basically putting tiny imperfections in the surface,
consider metal fatigue and the load on the workpiece when it is in
service. Better put, for example, when prepping a wing spar, it is
preferable to sand along the length (in the direction from wingtip to
wingtip, back and forth) rather than across/around it. Did I mention to
go light on the sanding pressure? You want to remove some oxides and
lightly scuff up the surface.

I find that after scuffing (on steel too) there is a very fine dust that
takes a few wipes to clean off. My favorite to clean this is denatured
alcohol on paper towel.

Oh... since I brought up steel, it initially takes some serious cleaning
up that is way beyond what I have the patience to do by hand.