Kydex is Weird stuff.
It's tough finding anything solvent related that will bite into it.
Even Methylene Chloride won't really bite it.
Plus it's got that molded texture pebble grain pattern so it can only be lightly surface sanded a little bit.
Not sure anything is really going to adhere to it with a solid "coating to substrate" bond but, your method probably has the best chance.
Much depends on if he is just painting one holster or wants to start making Kydex holsters and say...start airbrushing designs or camouflage or whatever on them.
For just doing one holster...my suggestion would be...Buy the exact Kydex color you want first and start over.
