Blisters in almost all cases are due to the escape of moisture that is present in all wood, new or old. New wood is, of course, more liable than old to give off moisture, and the paint to become more blistered; but old wood will show the same effet if exposed to the beat of the sun. It may be that exposure to the sun is the cause of the paint blistering on this particular door, and in that case the only remedy is to hang over it a, kind of sun-blind, made of plain or striped canvas, during the summer months. This is a very general practice in the London suburbs, and is found to be the best protective. If the door is to be repainted, then see that the work is done in dry weather and with dry brushes. The old paint will have to be burnt off, and more turps and less oil may be used with advantage in mixing the new paint, as a more porous film of paint will in this way be obtained.