When I say polishing jigs, I should clarify myself. I don't mean to polish them till they're baby smooth. What I do is this. Once all of my jigs are poured I then do the following. I cut off all of my sprues with side cutters. The reason I don't twist them off, like some do is because if you twist them off (depending on lead composition) you may get divots in the head. If you get a divot that is too deep, you will almost never polish (file) it out without taking too much lead off. Anyway once my sprues are cut, I then take a smaller pair of gate shear and cut off the remainder of the sprue left. Next I take a bastard cut half round file and round out the left over sprue, until it conforms to the jig body. If there are any other marks or nubs on the jig head, I file it down. That's it .When the jig is done, it is really not smooth, it has file marks from the file I used. However paint will have something nice to grip to, and also any and all powder paint I've used covers up the jig perfectly.
As far as powder paint being old. I have powder paint over 10 years old. if you keep it dry , out of the sun, heat and temp changes it will be fine. I can't comment however on Boss Armour paint, since I've never used it.
Anyway this is what I do. Everyone on here has their own method, so I'm not saying my way is the right way but it works for me.