I have an upper that has some deep gouging in the throat where the barrel extension slides in (see photo below).  I'm concerned about galvanic corrosion developing there due to the direct contact between the steel extension and the bare aluminum.  Any suggestions on what to use to cover that area so it doesn't corrode?

I've got Aeroshell grease for the barrel nut threads and I think that can help prevent galvanic corrosion, but I don't know if it would be good enough. It also seems like a bad idea to put grease between the extension and the receiver since I don't want to encourage movement between those two (after it is assembled).

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And NO, I didn't cause that damage. I bought the upper on the EE and the seller didn't tell me about it (the seller did make things right with me).

Are you going to keep the rifle exposed to salt air and humid conditions?  If not, don't worry about.  Put some Aeroshell grease on it and call it a day.  All this galvanic corrosion talk is IMO much ado about nothing 99% of the time.  I have dozens of ARs with the front of the receiver turned or lapped and the bare aluminum comes into contact with the steel barrel nut.  I tear things down on a regular basis because I can never leave things alone, and I have never seen even the slightest hint of corrosion.  But I keep my weapons in a dry place and keep them oiled.  They do not get neglected or ever over heated from mag dumps, so they are certainly never exposed to salty conditions, not here in the midwest.  Humid, yes, but never salty.

Birchwood  casey aluminum  black is what you seek.

Alodine is an option also if you're that worried about it.

This. I can't tell you how many times Alumiblack has saved the day for me, whether it's freshening up salty parts, or hiding scratches or gouges on parts I didn't plan on painting. Works great on RAS Rails, receivers, buffer tubes, etc and actually works on steel parts in a pinch as well.

Thanks everyone. I really appreciate the very helpful advice.

I use bearing lock on the receiver to barrel extension fit. That would seal it.

Thanks!  I need some, too!