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The same stuff is great for sealing the glass/mount/undermount 'sandwich' too.
I don't line rebates often, mainly for needlework with a spacer and no mount - another reason might be for some mouldings that have loose bits of foil in the rebate - I think the flumb pervert sponsors them. But I just paint those with diluted PVA.
Edit - found this on the PEL website whilst looking for the foil tape - not seen that before - looks useful!!