Hi, first post.  

Built two AR's so far, first time I touched an AR style since I was in the Army, close to thirty years ago.  I'll post pics of them a little later.

Anyway, my first two were kit guns, but I want to start with a stripped upper and lower and build it my way.

My question is, if I pick up a raw (bare aluminum) upper receiver, what would be the best finish to use on it?  Specifically, on the inside of the receiver.  I don't want to waste a perfectly good receiver by gumming it up with the wrong finish.  At the same time, I don't really have the ability to bake on a finish.  Any suggestions?

I'm okay with masking off the inside surfaces of the upper and spraying it down with Aluma-Hyde II or something like that, but is that the best option?

Thanks in advance!

Is there a specific reason you are looking at a bare aluminum upper?

Personally I would get one that is already hard-anodized.  It is the best finish for durability and wear because it becomes part of the outer layer of the aluminum.  Otherwise I would cerakote or maybe try DIY anodizing (which has it's own unique challenges).  
Painting bare aluminum is not my first choice and won't last more than a couple minutes inside the receiver.  The prep needed for bare aluminum, whether you paint, cerakote, or anodize, is not going to save you any time or money over getting a factory hard-anodized upper.

I was going to use the bare aluminum upper and lower because a friend of mine has a set he was willing to part with for a very reasonable price.  But, if it's going to be that much of a pain, maybe I'm better off spending the extra money.

I know enough about anodizing to know I don't want to try doing it at home.  

Thanks for the info!

I'd go with Cerakote.

Aluma-Hyde II is fine if it is all you can work with. A light coat inside will work as well.

KG GunKote, but a pain in the azz to apply.

Surface has to be abrasive blasted (not glass beaded),
degreased in clean acetone several times,
degassed by heating to make damn sure any solvent still in the pores of the aluminum is gone,
sprayed on while the piece is still above 100°f,
then baked at like 300° for an hour, while it puts off the most obnoxious fumes.
And doing the BCG bore will be a challenge.

But it is a very hard surface once done.

I only do in the summer when I can well ventilate the house....
Oh, and if you have a woman in the house, make sure she is gone when you do it

Personally, I would just pay someone to hard anodize your cheap receiver set.
But might not be so cheap by the time you finish.