Join Date: Dec 2005
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I would start by soaking them in WD40 or Liquid Wrench overnight, then try to get some leverage--e.g the right size screwdriver gripped in a large vise grip--push down on the screwdriver while turning the screwdriver with the vise grip. If that still doesnt work, I would add a 2 - 3 foot pipe sleave over the pipe wrench to get more leverage. After that, I would try applying some heat, though carefully to not damage the surrounding gelcoat.
Others may have some better ideas.