I would second every thing that Larry says in his post. I would emphaise that epoxy filler (I like to mix my own with West Epoxy and high density filler for the first application and West Epoxy with microlight for the final surface fairing.) must be used--never Bondo or such products. I have not used the vinyl ester products. A friend started, and eventually 3 M sent out a team to help--did a great job--and the product was withdrawn from the amateur market for a peroid of time. The second recommendation is to use air tools. They are much lighter and easier to use. I use a die grinder--in fact they are so cheap, that I use two--one with a carbide round bit for the small blister grind out. The second with the 3 M 2" 36 grit discs in the chuck, for medium size. The large ones I use an air powered 4 to 4 1/2" grinder. For the final a high quality orbital 6" auto finish sander--and at times a straight line sander is necessary. These tools are cheaper and far easier to use than the electric versions. Generally moderat size blisters are not cause for worry. They are best handled with each haul out--if you own the boat long enough you will probably cover the entire bottom with spots of epoxy cloth and resin. I have never seen blisters recurr in the same area where this type of repair has been done. I have seen several boats in Europe that were beyond repair--but this is extremely rare. Bob Austin