Keg has coating on inside

[MisterMike]

Hi all, I got an old keg today from a farmer, I was trying to make a simple rims set up like I've seen on YouTube Robert Baldwin's channel, I cut the top off the keg it's aluminium I didn't mind that too much but it has a light brown coating on the inside, it's stamped 1979, I've tried to researching it online it maybe an epoxy coating to help keep beer fresh, any ideas will if this is safe to use or just scrap it?

 

[The Baron]

Could be Beerstone build up over the years.
Wait for further opinions and if it is how to remove

 

[MisterMike]

Trying to clean it off with sandpaper.