I use a product called Dolphin Glaze, my local body shop put me onto it, its a bit on the expensive side
The Dolphin Glaze is a fine finishing glaze designed to eliminate pinholes and surface imperfections prior to painting. Its self-levelling formulation gives a smooth, ultra-fine finish which requires minimal sanding.
It is a two-component chemical liquid used for the final skimming of a polyester body filler putty repair or direct application onto minor imperfections on bodywork before overpainting. Also ideal for skimming over finished body filler, it offers high adhesion to galvanized, zinc-coated steel, cured paint and most plastics.
Aluminium, galvanised and zinc-coated substrates degrease and abrade with P120 paper. Do not apply over etch primer. Original paint surfaces (except for thermoplastic acrylics) – degrease and abrade with P180 grit paper. GRP – degrease and abrade with P80 grit paper.