ce-mix do a range of products.i used quick patch,multi bond and aqua stop.to colour it they make oxides .i used all these over polystyrene.mix up a slurry paint it on with a paint brush,many light coats.no need to seal it thats what the aqua stop doesthese were designed to do water tanks and ponds i think,cant remember what was on the containers.mine has been in with my africans for about 3 years.no problems at all. only issue i had,and it was my mistake,not the products. was put plenty of coats on to get a good thickness as when a rock or something damages it you get a white hole showing.i only did about 4 coats,in hindsight i should have doubled that number.all availiable at bunnings.