If you don't apply the gelcoat first, you will see the fibreglass, which is not very attractive.

Tooling gelcoat is harder and sometimes has a higher Tg.

The gelcoat I saw is transparent, I think because they need to show a CF feature on jet ski's surface.

In that case, its because the surface will be full of voids otherwise.

If it just to protect a surface purpose, could I use normal epoxy resin instead of  gel coat. 

It is also for cosmetic reasons. Probably a similar product to our GC50 or other polyester based clear gelcoats.   The surface wouldnt necessarily be full of voids if laminated properly.

Could also be done to provide UV protection and some scrape protection for the carbon.  A nice layer of gelcoat can have scratches sanded out where as with say an infused carbon panel, the layer of resin over the carbon is very thin and easy to wear through to.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thank you very big for your kindness help.

Hi, yes it is compatible with epoxy, but in my work, I don't use any topcoat/gelcoat I use only epoxy and then clear coat it.

~~~Everytime I hear sound of demoulding part I have eargasm~~~

Just like Warren said it gives you the possiblity to have some errors and fix them if needed...wouldn't be possible, or very difficult, with only CF. It does as well make the part UV-stable with a good gelcoat.