This period of blisters started during the 1970's oil crisis. It is because the quality of the resin took a nose dive at this point. There are several reasons why blisters show up, and they all combine poor workmanship with poor materials. That being said, if you get any boat that is old, if it doesn't have any sign of blistering, de-lamination, or cracking, it most likely will not develop these issues. If the hull is in good shape still with 40+ years of being in the water, chances are you will be fine. After having owned several sailboats at this point I prefer early 60's or late 50's boats because the resin was the best quality, they built to wood based dimensions back then so the boats are way over built (favorable characteristic in my eyes), and the hulls were uncored, as well as the decks. Yes this makes for a heavier boat, but you will be saved a lot of headache as most old boats have some level of deck intrusion and will need at least some level of deck repair if they have a core. My favorite point though is that they did not use insert pans for the interior yet, so you can lay out your boat however you want between the bulk heads and not jeopardize the rigidity of the vessel. Since GRP boats back in the early days were utilitarian optimized, near all of them are stout ocean going cruisers as opposed to day sailors like a clipper marine or bay-liner vessel. My old 1960 Rawson PH 30 weighs in at 7 tons unloaded, and is solid as a brick house. Worlds apart from the 71 Columbia 26, which was super easy to man handle, and could be sculled with the rudder in a calm. The big heavy boats take more precise judgement and actions or shit can go south a lot faster.
