Its not actually paint.
Its a waterproofing/windproofing agent for the cloth.
Helps turn the porous eaily soaked linen into a solid surface that wind goes over, and rain runs off.
It's generally clear or with a slight browish tinge, which is why most really early planes with just dope and no paint are an off white Linen colour.
it also shrinks the cloth as well making it nice and taut. Very handy in adding extra stress bracing to the frame as well.
Paint schemes would go on top of the dope.