I dropped off 2 aluminum 80s and 2 Steel 72s for hydro at Acme Fire Extinguisher Co. (1305 Fruitvale Ave,
Oakland, CA). I had them tumble one of the steel 72s as it had been without air for several years and I could see light rust. The hydro was $25 ea and tumbling was an extra $12. Overall I found them good folks to work with. They do not perform official visual inspections (meaning no VIS sticker or fill) but do inspect all tanks before hydro. Reason stated is they do not want the responsibility to install expensive chrome scuba valves and own any damage or scratch that might occur. When the LDS did the official VIS and fill they said all 4 tanks were exceptionally clean. Would use them again for hydro. I picked up some ceramic media at Harbor Freight that they sell as vibratory deburring. It is listed as "rust cutting abrasive resin" and is in the form of a 0.25" pyramid. ~$15 for 5 lbs. Have not tried it out yet. I need to remove an epoxy coating on two narrow necked tanks. Not sure if this will be aggressive enough. Easy enough to find out.
Side note: They said they no longer tumble small neck steel tanks because its a bear to get the tumbling media out of the tank.
“A skin diver is a fellow who pulls on a pair of fancy swimming trunks, some rubber fins, a diving mask and canvas gloves, then fills his lungs with air and noses down into the ocean looking for two fisted trouble.”