Can I seal concrete with Unibond?
 

I've just built myself a tiny pond and waterfall inside a tiny cave in a
cliff-face in my garden. The rock was very fractured and so I concreted the
base and water-course, but it still loses the water. I have a large amount
of Unibond and wondered if anyone can recommend using it for this job?
I do not want to keep fish or vegetation so that's not a problem.
Any advice appreciated. Chrisssss....... (North Wales, UK)

No, Unibond is definitely no use for this job. It is basically a PVA adhesive and is water based, in fact to thin Unibond you simply add water to it. It is non-waterproof so will not be up to sealing a concrete pond. What you need is some stuff called G4 which used to be made by (and probably still is) a firm called Bondaglass-Voss. It is a moisture-cured product (I think a polyurethane) which is used as a bonding coat between wood and fibreglass amongst its many other uses. I believe some people have also had success with Thompson's Water Seal, which is a very thin spirit-based penetrating sealer for masonry, canvas, etc.

Hope this helps,

Bill.