Hi Folks - Newbie warning!
First off I would like to say how informative this whole site is - well done for all your input.
Now - PU Foam is marvelous stuff we have; as has been said; used it over here for years but you have to know a few things about it. As you know a light misting with water helps it to set it better, but that doesn't always help as I shall explain. The stuff you and I use reacts with the moisture in the air, this starts as soon as it leaves the nozzle, the swelling of the foam is a result of CO2 being generated, a secondary reaction causes the stuff to harden from the outside in. This is what causes your foam to curl and shrink, if you make a big blob of the stuff and leave it a day or so then cut it in half the middle will still be wet and under pressure from the self generated gas! Remember it is closed cell foam, with large flat areas this distorts the sheet and causes it to bow either way. The fact that the outside has set prevents the inside from foaming and as CO2 is a natural dessicant there is no moisture for it to react with.
The solution to this is to poke a hole into the centre of the larger masses after it has set for say 8 hours or so, use a nail or screw driver, nothing large is needed, then sit back and watch how much foam will eventuall come out of the hole you made :eek1: For really big blobs make a couple of holes, then trim off the branches that are formed. Spraying water on it just makes the skin form quicker and with big blobs it makes the internal pressure worse.
I would like to say that I made a very impressive backdrop a few years back by using foam and Tuffa Rock, it is very popular over here, you get a sack full of lumps (56lb) put it preferably over someone else drain and blast it with a pressure washer (big mess warning) What you have left is a coralline skeleton from millions of years ago, this can be sawn up into slabs with a normal handsaw and foam glued to the sides and back of your tank. Mostly people do not paint it, firstly it darkens up after a few days anyway and the coralline seems to love the stuff regardless.
I also found out that it doesn't actually "stick" to glass, what holds it up (well IMHO) is the fact that it is firstly vacuum that keeps it attached to the glass, and once submerged the water capilaries in and holds it even better, once you tug at it it doesn't go back the same and a little fresh foam is need to get it going again.
As far as UV resistance goes i have some in the side of my house that has been is sunlight for at least 10 years and only the top 1/8" has discoloured, none of it has really disintegrated.
