Adrian gurgled happily, sounding much like they
were saying:

Captain Tolley's Creeping Crack Cure (no, seriously)

Good stuff, used it on an ordinary window here that leaked under gale
force driven rain. Took several applications but cured the leak.

Seconded!
Strange stuff - very, very 'thin' - but it finds its way into cracks and
has been 100% successfull (touching wood) at keeping the gentle West
Cork rain _outside_ my old moggie Traveller.

Have to admit, I've been wondering if it's just very thin PVA...

Who cares WHAT it actually is? The name alone is reason to buy it.