Just removed 6 frozen thru hulls. Simple dremmel tool with cutting disc make 4 cuts in the outer flange like you are cutting a pie carefull not to go too deep once the cuts are made drive a long handled screw under the cut pieces pry down. The pieces will break free and you can pull the thru
hull fitting out from the inside. Don't be fearfull had 6 out in 1/2 hour.