Sanding will not screw up the accuracy of the taper.
After working in a machine shop, I know for a fact. You can take some 320 grit to a piece of steel rotating at 1600 rpm and barely take it down .002" in half an hour.
Hand sanding with your finger and some 180 will not mess up your ball joint taper at all. Hell, the rougher you can get it the better. More rough-ness means the taper won't be so damned hard to break whenever you need to take the spindles off.
I'm sorry but my 30+ years experience as a Toolmaker & Precision Machinist tells me different. The purpose for the taper is so you have a solid joint, with NO wiggle room. Any movement in the joint will just lead to more movement over time. Also if you were to 'rough-up' the taper you won't get a good metal to metal seal which will allow water and other contaminants in and you'll run the risk of getting rust in there, then it'll never come apart.
Anyway... I did a little research, with no help from DJM, and found that 'Powder coat' is just paint applied differently. something I didn't know. One of the many advantages is you can get a heavy coat with one application, as we have here.
What I've decided to do is just get a good gel paint stripper and let it work over-night. I checked it this an real quick and it look to be working fine. I decided to get a environmentally friendly stripper so I could do it in the basement, where it's warn, and not have fumes running through the house. I've done so many stupid things when I was 'young & dumb' I can't afford to lose anymore brain cells!