You are completely free to feel however you want independently of your partner and in this instance I agree that you shouldn't allow him to paint your car.

Dabbling is a classic BPD characteristic, as is the impulsive buying. I once spent two grand on an easel, brushes, oil pants, and canvas. Its all still sitting in the closet after a failed dabble. This kind of behavior drives my wife NUTS, and knowing that I've tried to tone it down. But when she tells me I will fail (and the thing to note is that she doesn't say that I will fail, instead she expresses it much as you did, but what I hear is I will fail) I end up depressed, even if I was in a manic mood previously. I seem incapable of separating you may not be able to do that from you fail at everything which is what I believe of myself.

My wife and I have since agreed to take a 5 min separation anytime this happens (although it can last up to an hour, the main point is that we are not longer angry and can listen) then we sit down and tell each other what we meant and what we heard. We don't interrupt at any point until the other is finished. This usually results in me understanding that
1. she isn't leaving
2. she has a point
3. I completely missed the point
4. Had I listened I would have gotten it sooner
5. Its not that she doesn't believe I can't (in fact many time she thinks I can) but that she want a practice run that isn't nearly as costly.

The other thing you can do is to support the dabble in small ways that are not as expensive. In this instance models such as warhammer 40k or model cars, or even a $100 junker as a surprise gift.

Out of curiosity what else does he dabble in? Personally I'm interested in paleontology, physics, Chemistry, Theoretical math, philosophy, Theology, Magic the Gathering, Role playing, Miniature War games, caligraphy, Bonsai, Mechanics, Trapping, Bow making, Welding. The list goes on and on but if you need help with ways to support his hobbies that are inexpensive let me know.