Alright sorry for the delay in answering your questions.
Here is the background in my 4 foot tank looks good very happy
I built up layers of 60mm poly medium density sourced at placemakers,
glued with gorilla foam out of my builders kit.
2.Cut to fit tank with cut outs for filter in/out, heater. positioned heater close to power head so it draws water passed heater
3. Carved out rock shape's with my builders knife and gib keyhole saw.
made holes in heater area in the rock cracks and holes for filter, power head.(what a mess on a drop sheet but still every where!)
4. Plastered poly with CEMIX patch and CEMIX multibond mixed together
with coarse sand, Patch 3 parts, sand 2 parts and multibond watered down half/half. applied with brush and by hand with rubber gloves.
two coats all around.
5. Cut background up into 4 bits, cutting down the rock crack lines so to hide cut as good as possible, plaster cuts once done
6. Painted with CEMIX aquastop tinted with coloured oxides whatever colour you want two full coats and a few other partcoats mix of colours
flicked over top with brush.
7. mix some more patch, multibond, and a little aquastop not to much as I want to put some black oxide in to get as black as possible and aquastop will lighten this up
8. Soak the bits of rockwork for couple of weeks to let any excess lime bleed out.
9. Install in tank and enjoy keeping an eye on PH to make sure. I am still going to do daily water chances for a week to make sure.
Also I rang technical people at cemix and they said there should be no proplems with these products they recommended to put aquastop on to waterproof everything last as I have done above.