Blistering paint is usually a result of moisture and heat. Moisture under the surface causes paint to detach from the original structure and heat causes it to bubble. Some older homes have extensive problems with blistering. In such cases, the only way to truly guarantee good paint adhesion is to strip the paint in entire area and start with bare surface. Otherwise you are counting on primer/paint from long ago to maintain its bond with the underlying surface, which often it can no longer do. When blistering is a smaller problem, it is appropriate to simply scrape and sand loose paint in immediate area and apply appropriate bonding primer before top-coating (oil-based primers are best for wood).
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