just buy a 2 part polyester bodyfiller in a local diy store, make sure to seal al the bodyfiller with the pattern primer from EC before making a mould, 
Sorry I'm to late but vaseline or any other 'non-composites' related products have a high risk of failing so while reading through I kind of knew it was going to fail again.
Just do it the proper way, will cost a bit more, but on the other side you wont waste your time repairing errors and failures, so you're better of at the end