wall thickness with full fill?

[Kevin Johnson]

... Yes I like the idea of using modelling clay to make a water channel! I've used thin rubber hose before and pulled it out after the fill dries but never thought of clay!! how simple, thanks..

Another technique from the dental lab industry is to use wax. Similar products in use (grout/block-fill versus plaster/improved-stone). You can buy water soluble wax but I think utility wax rope would be easiest to work with. You will need to flush it out with boiling water, best laced with an emulsifier. Dental lab supply houses should have the wax and emulsifier.

[JWebb]

... Yes I like the idea of using modelling clay to make a water channel! I've used thin rubber hose before and pulled it out after the fill dries but never thought of clay!! how simple, thanks..

Another technique from the dental lab industry is to use wax. Similar products in use (grout/block-fill versus plaster/improved-stone). You can buy water soluble wax but I think utility wax rope would be easiest to work with. You will need to flush it out with boiling water, best laced with an emulsifier. Dental lab supply houses should have the wax and emulsifier.

Or carve out your shape in styrofoam as a plug, grout around it, and dissolve it out with brake clean.