Aluminum Oxide cleaning on a New Cast P3 (also, paint removing from two other bells)

Hello again! (second post here)

I come back with another question for those who know more on the maintenance and/or restoration of these beautiful horns. As the post title says, I am currently trying to clean off some thick, flaky, white aluminum oxide off of my P3 horn (namely the 1, 2 and 4 bell, as well as the manifold). I tried this morning only on the P4 bell with a solution that was mixed parts distilled water, vinegar, lemon juice, and baking soda. This worked in getting some of the lighter stuff off, but a lot of the harder oxide remained. (I let the horn sit submerged in the solution for roughly 2 hr 30 min).
So far, this is removing the lighter stuff, but it also seems to be staining the metal with a less than desirable color. So I pose this question: What would the best grit for sand blasting be for this? I wanted to know as I am still relatively knew to the restoration field, and wanted to no kill my only horn. I hope to hear back soon from anyone of you lovely people here!

(If needed, I can post a picture of the bells in question to show the oxide we are talking about. Also, would this blasting medium work not only in removing the oxide but in also removing some black paint off of two bells from Hornblasters?)

P.S. if this seems less than desirable to read, I am not good with typing out long post like this. Please bear with me!