I have a 9mm glock mag AR lower that the magwell was way too tight.  I sent it back to the original manufacture and they milled the magwell out so that my mag's will drop free.  they didn't (and I wasn't expecting them so since they said they don't do it in house) refinish the inside of the magwell.  Right now it is bare aluminum.  

What is the best and easiest way to refinish only the inside of the magwell with adding the lest thickness to it?  I don't really care about the looks but do care about protecting the aluminum.  It has a F1 pending on it and also it will be shot the majority of the time with a can.  That also means it will be cleaned a lot to remove the blowback buildup from both 9mm and 40sw.

Apply mild heat to the magwell with a heat gun, clean the surface with dawn and then 90% alcohol.  Once that's done, heat the surface again, wipe it down with alcohol one more time, heat again and then apply aluma-black to the uncoated surface and hit it again with the heat gun.  Let it sit for a day and treat again without cleaning, just warm it up again:
https://www.birchwoodtechnologies.com/Downloads/Blackening-and-Oxidizing-Concentrates/Aluminum-Zinc/6300_Aluma_Black_A14_Instructions.pdf

so that should be good to take the physical abuse of inserting mags and also the chemical abuse of cleaners?

thanks

It's not going to up to the same standards as hard anodized or Cerakote but if you are looking for an at home with no specialized tools solution that isn't spray paint then it's it.  Olympic Coatings is good to go with Cerakote but you'd have to send it to them.  I would assume they could mask and just to the magwell for you but if you still need to make your form 1 engraving marks, I would do that and send it to Olympic for a full Cerakote treatment along with the upper so they match reasonably well.

Ya the lower is already engraved but the f1 is going to pending for a bit.  

I am waiting on new century to release more uppers right now.  Need to decide on handguards also.  After that I may then go send it in then.

Thanks

Alumablack is a good base coat.  I would then go with KG- or Cera- Kote if you need a color coat.  Magwells that are tight are gonna just gonna wear.  I have a lower like that.  Just recoat it each time I re-furb the rifle.  No big deal.