No need to buy a special gel coat as a non surface sticky gel coat can be mixed by adding a liquid wax to the pigmented resin. The wax floats to the top, seals the surface and the resin goes hard underneath. You need very little of it as only 2 per cent does the trick. Supplies can be obtained from any good GRP supplier, find them on the web.
This technique called Flocoating is often used on the inside of hardtops or dinghies.
The alternative on small areas is to cover the surface with thin melanine whilst the resin goes off, you can even use resin resistant parcel tape but you will need to hand finish the surface to the correct shape.