An imperial Gallon gets you 128 fl oz so , if you assume a standard bikers helmet size at say a 1/4" thickness, you should be able to make two helmets or one very good one out of a gallon at 64floz per helmet .
Release agent - get into the habit of always using release agent because ultimately it preserves the life of your mold , as every cast you pull , the mold breaks down , buy the best casting compound you can afford as you want to replicate as much detail as possible , and the same should hold true for casting resin, clean and periodically rest your mold - Don't try to pull 50 helmets in a day , you'll end up with bad casts and no mold !! Take care of your mold and you should get 30 to 40 casts , make a fibreglass mother mold to extend your silicon mold's life. I'd do 5 or 6 as a maximum , remember most resin hardens by thermotropic action , that means heat - a lot of it ! Heat exposure over time will breakdown silicon even high temperature hot metal casting silicon. So rest your molds
and let them cool down between castings.
It is possible to cast over wet clay , but it's not recommended , too many things can go wrong and once you've poured your silicon you are committed !! use modelling clay or any other modelling compound eg super sculpy - plasticine etc these consistently give good results and you shouldn't go wrong , if you want a reuseable product Plasticine is your best bet .
