Agree with Ralph. After all, it's your work, not some conservator's. He should be accommodating you, not you him. At least if you're at the level that it matters at all. Fortunately, I'm not.
Can you imagine? A museum conservator approaches the man holding the brush and says, "I don't care how subtle her smile is going to be. If you paint her on that particular type of wood I'll have nothing to do with it. Storing it will be just too much of a pain in the ass for me."
So a miffed Leo replies, "Forget it then, pal. You don't deserve a painting of her anyway. I'll go find another subject."
And the world would have been better for that exchange exactly how?