kiwikx500 ,
i just had my radiator welded up last month ,it was leaking also out of a crack ..
i went to an official high recommended radiator-repair/restore shop and asked this same question.
that owner who also builds for paris-dakar rally vehicles stated very clear..
never ever use this clogging up crap in a motor !!! it fills the cracks and wholes ,but it also is clogging up al the narrowing routes of the coolant ..
leaving you with a messed up engine that no longer has a free coolant flow
thats just like humans with by-pass for their hearts .
i took his advice immediately and had still no single moment of trouble with the rad anymore