Blistering

Cause
Generally blistering  is caused by moisture ingress into the underlying substrate. In time this builds up behind the paint coating resulting in blisters forming and in many cases, when broken, contain water.

In other cases blisters can form in and around wood knots and these can be as a result of gases from  hardwood knots and sap.

Solution

The cause of the water ingress should be determined and rectified.

Note: Where wood knots are present then these should be either cut out and replaced with sound wood or two pack wood filler  and suitable primer. For hardwood surfaces a spirit based aluminium wood primer could be used.

Shellac knotting will not resolve blistering problems as it is only designed to isolate the tannic acid, preventing it coming into contact with oil based alkyd paints to preventing discolouration. (see yellowing)

Complete removal of all the existing coatings and repaint with a standard system for wood.