For waterproofing I highly recommend using a two-pack epoxy as used for boat building/finishing. OK, it's expensive, but if you build up 2-3 layers you will only do it once!! I built my sink out of 20mm external ply, with the box glued together with high performance silicon glue (I think it's silicon based - comes in a mastic gun tube and says it will bond anything to anything and be waterproof) and then screwed every 100mm. 2 good coats of epoxy and it looks like it should last for ever.