OIL FURNACE HELP

Sounds like the exhaust fan you're running for the spraying is probably pulling a draft back through the chimney. Could be dangerous. To get the pressure inside and out to be more equal you either need more air intake or less air being exhausted. You could put some yarn strings near the furnace door or the hole in that door to see which way the air is flowing when your exhaust fans are running.

Personally, I'd either isolate the furnace from the ventilated space you're painting in, or turn off the furnace while the exhaust fans are running.