Hi!
I will CNC cut a plug from Polystyrene foam and will buy the Easycomposites  "Uni-Mould Complete Mould Making Kit "
My question is:
How can I get a smooth surface at the finished foam-plug before I put gelcoat on?
Is there some kind of product that would be suitable for this?


The plug is 1800 mm long and 600 mm wide.

Have a great day!
Regards.
Joel

Ok a bit bigger then.

It just seems that since you have CNC to accurately make a nice pattern that a firmer material will better replicate the CNC. 
With some general strength better than a typical polystyrene. Maybe urethane foams? or even PVC foam$? Then it safer and easier to do hand work on thereafter and without fibreglass even. .

If you were to glass a polystrene foam you could try the really light tissue type fibreglass wet out with epoxy resin, let cure and then the epoxy fairing paste you are thinking of.
There are additives to make your own fairing type paste like Q cells and microballoon powders to add but these are different to the ones that make thicker resin. A thicker resin is like making a glue and these glue additives make a resin that is denser and very hard to sand. The sanding fairing type filler must be used or you can buy epoxy faring pastes as A + B kits ready to go. Resin suppliers like easy composites could have this ? or any marine shop can be a good source for these things.

 

Thanks Fasta,
A very good advice about not using dens epoxy. It would be a nightmare to sand down.

I will definatly go for "epoxy faring pastes". I found this at easycomposites: 
https://www.easycomposites.co.uk/emp60-epoxy-moulding-paste

And I will go and see what kind of foams they have in the local shop. PVC or urethane foam sound like a good alternative to the standard white 
polystyrene


Thanks a lot!