Thanks for all the positive feed back
To answer a few of the questions above.....
My build is in the Large Reefs forum called UK 180g Reef.
Pushing anything into the foam is hard - it takes about 3 hours to hard but skins over in seconds so sand just falls off. Gravel can sort of be pushed in while its very wet (first seconds after applying) but it tends to break the foam down and is just a bit of a mess really (and i wanted resin used as explained below)
The foam floats like crazy - i was amazed when i put a lump weighing 15kg in large tub of water as i was interested how heavy it was submerged (i assumed it would loose half its eight under water).....it floats like a boat!
Thats why i have to apply the foam to cover any cable ties, link sections of rock together (to give the structure stability and also make it look very organic instead of like a pile of individual rocks and then cut of any excess - i must hack away at least 50% of what i apply before it will sink.
This is the middle section above with foam just applied:
At first i thought the round shapes of the foam looked good and was annoyed to hack them away to get it to sink - but once coated in gravel the harsh edges of the trimmed foam look far better (and it means there is more real rock on display)
The only way of shifting water behind the structure is via the return pipe that exits behind this....however, i dont believe that muck collecting behind rocks is a big deal if you have the right clean up crew (just my opinion) and also this rock is many times more "gappy" that a traditional stacking of rock. Good flow in the display will cause movement back there simply because the holes between the rear of the rock and the front are so well spaced (although they are not obvious due to the construction - it would be easy for any large tang to get between the front and rear of the rock in many different places - very open)
Does it rot? .....i hope not! All i have read seems to indicate that water is not an issue but UV light might be enough to cause it to go brittle - hence the epoxy and gravel. This should be enough to keep it protected....but if its not it shouldn't matter - the foam is not holding the rock up - the cable ties do that. If the foam became so brittle it broke free of the points in joins rock to rock the rock wouldn't fall down, it would just become loose on its ties and wobble if pushed (not ideal, but no big deal)
I am pretty confident it will be fine for years (the epoxy coat is like 1mm of plastic covering all the foam + gravel)
I will have LR in one of my sump tanks (i have a sump shed)...BUT i believe the rock here will be VERY useful for filtration. A few UK reefers have said "looks nice, but no filtering"...which is wrong. Of course the LR is dead at the moment and will need to be given time to become live but when it does only about 15% of its surface is covered by foam - and the open layout means it is exposed to flow far better than a normal pile of rocks.....it wont be my only filtration - but it will work.
Heres a tank shot showing the very first bit (which i had to re-do as it floated!) and the layout of the wall around the weir......loads of space left in front for the islands.
Just to add - although i mention tangs above i dont plan any large fish in this, just LOTS of small ones (big fish ruin the aesthetics of a short tank IMO)...and my last tank had 3 gem tangs so i kind of done the tang thing anyway