There is mechanical repair option that works in some cases-- if you can reach in through the fill hole and you can reach to get nuts onto through-bolts that hold a repair pressure plate (brass or SST) over the cracked area. the seal is made by capturing a rubber gasket between the tank and the repair pressure plate. the more screws the better (up to a point)and these are SST also.
IMPORTANT: Also be sure to drill a small "crack stop" hole at the end of the crack before you effect a repair by any method. This little hole (.093"??)will spread out the stresses that are causing the crack and will very likely dispel any further cracking. I am not making this up it really will help.
I did a mechanical fix once years ago on M382 #002, which has a single big poly tank on starboard but NOT glassed in.
The crack was near the vent I think but in the flat, and it seemed to work for a long time. Soon I will need to repeat it (or I have another) so the welder idea seems interesting if we can find someone near Baltimore/Annapolis to do that. Also what about that tape which sounds like an educated big brother of Duct Tape (name of Eternabond makes me think they seal caskets with it, and named by a funeral director), and I want some!
Yes keep us other leakers advised of your progress.(mine is up high and only bothers me on a big heel on port tack, and when I am filling the tank. Better yet, during the winter it doesn't bother me at all...
I agree nothing/no adhesive will work on polyethylene (this is called the high density variety I think), so do not bother.(maybe as a gasket, use RTV bathtub stuff under a pressure plate.) Also I would not want the urethane in 5200 in my drinking water by any means. carcino-something I would guess. But maybe you could sue 3M for not telling you to not do it, this often works on cigarettes. Then you can buy us all new tanks? (this is meant to be funny)
Carl
SV Courante Annapolis