The blistering of paint is caused by the presence of water either in the paint or in the substance to which the paint is applied, greatly aggravated by the action of the sun upon the door. The old paint should be burned off with a spirit lamp, and the surface of the door well rubbed down with glasspaper. Then give a priming coat made of 21b. of white lead, 3 oz. of red lead, and 3 oz. of yellow ochre (note that the red lead is a drier). Thin with one-third raw oil and two-thirds turpentine. Finish in any desired colour, vising as little oil as possible, or turpentine instead of oil. Varnish on a dry day with a good varnish. It is better not to buy the varnish from an oilshop. Clean all water out of the brush before painting; a dirty brush - i.e. one with water in it - is often the cause of paint blistering.