Correcting the cause of paint blisters

Q. My question concerns paint blisters on the sunny side of a 39-year-old clapboard wall which has been painted and repainted with oil-base material. Two coats of latex have been applied in the last nine years. I suspect that the blisters are caused by insulation which was blown into the wall cavities two years ago. The blistering gets a little worse each summer. Behind the blisters is bare wood. What is the remedy? Kent R. Spelman Independence, Ohio

A. We doubt that the paint blisters are due to the wall insulation or to the latex over the oil-base paint.

The fact that bare wood shows under the blisters indicates that the original oil paint or primer has lost its adhesion to the wood substrate. A professional on-site inspection to verify this opinion is recommended.

Paint blisters on a nearly 40-year-old clapboard house are not uncommon.

Why? Additional coats of paint applied to the surface shrink when drying. A tremendous "pull" is thus exerted on the old surface.

Heat from the sun tends to further aggravate the problem by vaporizing any moisture in the wood and thus accelerating the formation of the blisters.

Ultimate correction lies in removing all the old paint (no fun) down to the bare wood. The sequence then is to prime the surface with an oil-base undercoater, followed with a top coat in a finish of your choice.

Admittedly, this program is not the easiest way to paint a house. Ho wever, until it is followed, there is no sure way to know when or if new paint blisters will appear.