G4

G4 is a brand name of a popular pond  / pool sealant, other makes are available. It is basically a waterproof resin for use on concrete, you should always read the manufactures instructions, it is available form most good aquatic stores. (also on line)

Please also note that G4 is a pond sealant, it will not harm aquatic life, other sealants not designed for ponds may. (So don't say i didn't warn you)

Basically after repairing your concrete pond or before you fill it if you have just built a new pool you first paint from left to right, allow to thoroughly dry then apply a 2nd coat going up and down You must always follow the manufacturers instructions including drying time between coats

We do not recommend any where in particular to get G4 so please do not ask.

see here