overlaying epoxy resin on PLA
I have used Styrol based Polyester resins on prints and they created the usual stench as well as a surefire bond and it is easily useable with unstructured fiberglass, as that fiberglass has usually a binder that will react with the styrol and bond the mat.

Epoxy resins also bond nicely to PLA and don't have the styrol smell, but they are not bonding that nicely to normal fiberglass, you want to use them with fiberglass weave.

Either resin is a quite viscous fluid. When you cast a flat surface, it will try to smooth out to a good degree under gravity. When coating a curved surface, you should make sure to align it in way that the lowest point is either the top of the dome or the lower edge, so it settles equally. You can aid in this process by providing heat as this will lower viscosity. An airstream will also aid as it presses onto the surface helping to smooth out unevenness.