[Tom Edgerton]

Marvin--

Velvet's good of course, but anything that kills the reflection from the wall will work, if money's tight. Even with a lens hood, there's junk in the air between the lens and the painting. As you know, down here in the South, it's WATER!
(I'll see you or your twin in DC...)

Everyone--

What, no lights???

I'm not sure about the suggestion of shooting in full sun--I'd bet it will glare on varnish. But hey, try anything.

I used to hang my paintings on a nail on the shadow side of an outside storage shed and shoot without lights. Meter over the surface of the painting as described to make sure the light is even. The only drawback is that plants, sky, etc. will reflect in additional colors. Maybe you can correct for this in Photoshop.

Bart Lindstrom used to open the garage door when the sun was on the other side of the house, so the door was in shadow, and set up his easel just inside the door and shoot with ambient daylight. He may still do it for all I know. It would eliminate the reflection of light from a wall behind the painting. Duck down to make sure you're not casting your own shadow on the painting. Also worth a try.

XXOO--TE