One of the guys I work with is now set up to machine 80% lowers.

Basically asked- Now that the Lower is milled out, what next? How is the inside treated? Anodized? Ceracote? Or something else...

Thanks in advance for the replies and the shared experience!

You will probably want to hard anodize it.

I have found that it is nearly impossible as an individual to get anodize done for a firearm that is not on someone's books to transfer in/out to the finisher. Local to me is the people who do sig's work and not a chance. At least in the area I live a dozen contacts with anodizers was no luck either too low volume or no go on a firearm. And just the same for ceracote, nobody who does firearms coating wants to log in an unserialized lower. I think the best strategy based on what I know is to start with an anodized 80%. Myself I decided to anodize it myself which is not exactly the same as a professional t3 job but good enough for this use. As would be any al coating of a similar nature. In reality the bare al will be fine as far it being a lower. In addition for the OP someone else milling an 80% lower that's not theirs is a really really big problem for you and them. But I'm not sure if that was what was implied or not?

Thanks for the info.
Looks like he's going to ceracote it on his own.

In a perfect world you would anodize it, but you'll need to put a serial number on it if you plan on sending it out to a company that does them.

There's just no comparison in terms of hardness and wear resistant properties. If this is something that may bother you then I'd just stick to factory made lowers, cheaper anyway.

At least in my area adding a serial number made no difference (I was unwilling to ship mine). Several companies make an already anodized 80%, doing it again I'd start with one of those. I wouldn't worry in any way about the internal situation on anodization, but its nice externally.

"Alodine" will get you some corrosion resistance, it's used as a surface prep for aircraft aluminum.

As for the aesthetics side of it,
maybe a used toaster oven/ kitchen oven and ceracoat it yourself with a "flocking" technique after masking off everything.
I have no idea if it's feasible, but its such a small portion of an already small object.