I have a concrete pond, the corners have been sealed with a black paint-on
substance, then a layer of what looks and feels like thick alu foil, then
more of the black stuff painted on.
The black stuff is very thin, more like a coat of paint than a rubberised
sealant.

Some of the foil type stuff has come away, I want to seal the corners in the
same manner. Problem is, I don't know what either the black substance or the
foil substance are.

Anyone know if this is standard stuff, or what?

Pete