Yes. Fairing compound (a.k.a. filler). It is basically like plaster for boats. There is a wide variety available. Speak to your local fibreglass suppliers and they will recommend a product (or products) that will be appropriate for your application.
Basically, you will need (1) Resin, and (2) Fairing compound. The resin will be 2-part. Epoxy
is probably best. The fairing compound is like a powder that you stir into your resin to make a thick paste (kinda like plaster). Typically the compound will be microsocpic balloons or fibres (or a combination of both).
Basically, you sand the fibreglass surface, then clean it (typically with acetone). Then you mix up your fairing compound; which, because it uses 2-part resin, will "go-off" (harden) fairly quickly, so only mix relatively small batches. You screed your filler
You apply the fairing compound over your fibreglass surface, typically with a plasterer's trowel. Get it as smooth and even as you can. Let it go-off (harden) completely, and then you can sand it (typically with an orbital or palm sander, but hand sanding
also works) to a perfectly smooth surface. The thing is, though, that it is quite hard work to sand, so you really want to get as close to an even surface when you lay it on, otherwise you will spend days sanding
it back to "fair".
After the fairing compound has gone off, you may need to re-fair it with more compound to fill low areas, rather than sanding the whole thing back to the lowest point.
Once you have the surface-filled and faired/sanded to your satisfaction, you can finish it off with an appropriate high build primer and top-coat.