hi oddjob a good product is acrylic gel sold at art stores the one i have used is made by liquitex and it is a gloss gel medium,dont be put off by its appearance in the jar (a white coloured paste gel) as it dries crystal clear,you can also pre colour it if you want or use colour on you base and just spread gel over it doesnt go off quick so you have plenty of time to spread,form waves or whatever,also read your question re snow i have used baking soda firs used it on a figure diorama 8 years ago and it hasnt yellowed yet or been attacked by insects plus it is cheap also i seled mine with hairspray and have had success sealing with gunze sangyo flat clear