120 grit Aluminum Oxide.

To answer the questions in general...

There's two thought processes on this. The first is that the anodizing with a light blast over it will offer more protection and hardening of the receiver prior to the cerakote. It also fights corrosion and wear.

Alternatively, blast it all off and while the dye and coating may be gone, they really only enter the aluminum about a thousandth. Give or take. So the cerakote would then provide it's benefits, corrosion resistance, chemical resistance, abrasion, etc... Once the anodized layer is gone or blaster down.

Now, keep in mind there are different types of anodizing as well. So some of the above may vary.

We do hundreds of receivers each week for both customers and lower receiver sales + cerakoting and have NEVER had a problem. The cerakote is extremely tough and does what it was designed to do. The blasting is done to change the surface texture of the material, so you may find you don't see a change except maybe a cloudy look to the part. Either way it'll all work as it should even if you don't take it down to the raw receiver.

Finally, we do tons of raw receivers that have never been anodized as well. Never seen a problem at all with them after coating, and never had a complaint. We do however do our bast to blast it to an even finish in order to not have any color variations.