I like the idea of a white background to help illuminate a dark under-board area, but I strongly advise adding a couple of coats of clear varnish if you can, to seal the emulsion and the boards, because emulsion is water based which is why its so easy to clean the brush's off under the tap afterwards, so your boards will still be affected by the atmosphere. Perhaps opening the garage door and using a mask might help, or even paying a small child to inhale the fumes instead