After a few more tests I'm starting to get the result I want with the foam. I have been using a lot less of the mix and adding more pigment as it thickens it up. It's pretty much the same as the last lot but is curing better and a lot less brittle. More test with ensue.
I have also got some liquid latex to try and skin the foam. I did a first try with this by just brushing it on, giving it a few coats. It turned out quite good but I am going to try and do some dipping tests.
I intended to try doing a layer of liquid latex on the mold and then the foam but ended up testing out whether I could actually cast with the latex. I added some of the thickener to the latex and poured that into one half of the mold and then added more thickener to the latex to make a paste like consistency and added that to the other half of the mold.
The half with the thinner latex took around 3-4 hours to dry where as the other half is still drying.
I think another lot of tests is in order.