Hi,

I need to sandblast my moulds for matte finish, but after that I have lots of pinholes.
I used spraying.

Does brushing gelcoat to mould avoid pinholes? Or what can be pinhole free mould gelcoat, not only surface but entire coat?

It's likely to be porosity, which is just a problem associated with polyester and vinyl ester gelcoats. Vinyl ester should be a bit better than polyester, so if you used polyester then it might be worth trying vinyl.

Does vacuum help? I think it's too viscous for vacuum