When using polyester resins and csm (unimould system) the best way to fill the corners is a narrow brush, or ideally a corner roller tool. Csm gets down in every corner if you are careful.

Hello Warren, thanks for your reply.  I should have been more specific and said that this is during the production of the mould, after the Gelcoat has been applied.  Just wondering about the best method to fill in corners with the Unimould system.  I am guessing the same applies, but should it be before or after the main coat of coupling, or does it make no difference?

First of all, heavily key/grind back the gel where you want to fill in.  You want the finish to be as rough as possible to aid adhesion.  

Making your own paste using small bits of CSM is fine.  Just be aware that if you cast a thick layer of resin, it can overheat as it cures so you may want to consider building it up in layers or reducing the catalyst ratio of the MEKP slightly to slow things down a little.

Obviously once almost level, you can fill in the surface with some fresh Unimould Gelcoat. However you will need to use some Solution MW Wax Gelcoat Additive to prevent the gelcoat from having a tacky surface.  

Once dry all that is left is to sand smooth the gel and polish it up to a high gloss to blend in with the rest of the mould.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

If I want to fill a square corner on a mould tool using Uni-Mould, should I use a Coupling coat / CSM "paste" at the corner?  And if so do I add the paste prior to the main coupling coat application or after?  Or is there a better way?