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I have an aluminum part that is shaped like a picture frame. It is 12
inches square and inch thick. It has a 1/2'x1/2" rebate (where the
picture would go if it were a picture frame). Two cutters were used to
make the rebate. The finish cut was made with a larger diameter
cutter(1") than previous cuts (1/2"), and to maintain the smaller
corner radii, the final cut left a small area in the corners that
protrudes above the finished surface by as much as .015 to .020.

What would be a good way to rework some of these parts without sending
them to a machine shop? Could I use a laminate trimmer or router with
some kind of end mill?

I can't visualise from your description exactly what shape the bits you need
to remove are nor do you say what tolerances you are trying to achieve.
However, a skilled operator can work to pretty fine tolerances by hand with
dremel type equipment. Basically what we do when porting cylinder heads. For
very fine work in aluminium I find it easier to use a mounted stone
lubricated with kerosene than a carbide cutter. You can shave away a couple
of thousandths of an inch at a time with care with these.

Pink aluminium oxide ones meant for grinding cast iron are the ones.
Something like a 10mm diameter cylindrical one on a 3mm shank lets you get
into fiddly corners. Used dry on aluminium the pores clog almost instantly
but keep them wet and they can remove material surprisingly well. For flat
surfaces a coarse oilstone also well lubricated with kerosene will remove
high spots in aluminium very well.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)