Blisters are really not the problem everyone makes them out to be. Most of the hype was created by companies selling repair regimes. Water is always flowing in and out of your gelcoat. Blisters are just that, a pocket of water between the gel coat and the glass. The glass is not affected by the water, as long as it is sound.
In reality it is simply a image thing.
If you don''t believe me ask a Surveyor. I currently have about 12 blisters on the bottom of my boat and when refinancing neither the bank or insurance company cared one bit about the blisters. Why, they don''t affect the structural soundness of the boat. Maybe the resale value for cosmetic reasons, but not the sea worthyness of the boat.
The best approach is that every time you do a bottom job, fix the worst blisters, budget considered and don''t worry about the rest. Over time you may solve the blister problem without breaking the bank.