Unless you really need the transparent qualities of polyester you are much better off using a polyurethane casting resin. Polyurethane resins have far less shrinkage (usually less than 1%), are easier to use, less stinky, and work better in silicone molds than polyesters. They are more expensive but you get what you pay for.You can still mix marble dust into it if you wish. There are clear polyurethanes as well though they are a little more trouble to work with.
Check out Reynoldsadvancedmaterials.com for their Smooth One resins. There's lots of good information on their site too.
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As Cavx said, a little more information from you, including photos would help us help you. The size of the casting alone can have a huge effect on the results and recomendations. (Larger parts, more exotherm (heat), = higher shrinkage.)
Good luck!
Check out Reynoldsadvancedmaterials.com for their Smooth One resins. There's lots of good information on their site too.
'
As Cavx said, a little more information from you, including photos would help us help you. The size of the casting alone can have a huge effect on the results and recomendations. (Larger parts, more exotherm (heat), = higher shrinkage.)
Good luck!