+xFumed silica is virtually essential to make a thixotropic paste. Calcium carbonate will increase the hardness and gloss retention. Graphite powder will give you a nice slick dark grey/black surface. Aluminium powder will help with temperature distribution. There's quite a few others, but those are the ones I would probably start with.
My advice - it is cheaper and easier to buy the proper stuff. By the time you've bought all these additives, you won't be saving any money. And then to blend gelcoats becomes quite a time consuming task, and to get it the same every time means keeping records of your recipes. Above all, it just doesn't work as well. The porosity is higher, so the gelcoat is weaker and you can end up with quite high surface porosity. It won't ever polish up as nicely, the gloss retention and polish-ability just isn't as good as the proper stuff.
For one off stuff, I would maybe think about using mostly fumed silica with maybe some graphite powder or black pigment just to give it some colour. But you will find it won't be a lot cheaper than proper tooling gelcoat.
Thanks, very kind as usual.
I agree,.this is sort of "emergency" because in this Xmas time most warehouse are closed.
I had on order the gelcoat from Ec but brexit stepped in.