In a pinch, I can successfully photograph my paintings out of doors.
I'd nail some black non reflective cloth (or something a dark neutral color) on the north side of the house, garage, barn, whatever. The point is to be in open shade so there will be even illumination and no glare.
Hang the painting at eye level over the black cloth so that the cloth is blocking out the background. Having the painting hang vertically and the camera lens aimed at dead center will help eliminate distortion.
Stand in the shadow cast by the building so that no sunlight will be directed at the camera lens.
Set the aperture of your camera if possible to the highest number you can in order to record as much detail as possible. Adjust your White Balance if you have that option.
Use a tripod to avoid any blur. With north light, there is no glare and your colors should be accurate.