Re: Removing old thru-hulls
For a non-recessed mushroom profile (external) through hull .... Use an angle grinder from the outside of the hull, grind off until flat with the hull, then punch the remaining part through. Use a 'deep socket' of the same OD as slightly smaller than the 'hole' as the 'punch' and use a manly sized hammer.
If recessed and flush to the hull profile through hull .... cut it from the 'inside'.
Use a heavily water soaked turkish towel wrapped around the valve to help cool and prevent heat damage to the FRG, etc. Don't grind away in one 'fell swoop' but allow intervals of good heat transfer into the wet towel, so that you dont overheat/damage the FRG.