Having fixed a mould with the unimold gel coat, results were good but it's really hard work sanding it back! 

Yes you need to use the additive or you could cover the resin with release/flash tape.

Just wondering,

If it is a minor chip or void in the mould, is the addition of a wax additive to the tooling gel coat necessary? Will a vinyl ester tooling gel coat not harden completely?

Thanks in advance,
Jady

Either use the original gel coat with some wax add and sand back through the grits to a high finish, or I have had a lot of success using durabuild HHR putty 

Hi all