The grey scotchbrite pads are not as severe as the red ones and should scuff the surface enough. I have the mirka pads but they are small and not easy to hold for scuffing large surfaces. Very fine grain sponge pads from 3M work well and are about A5 size and easy to hold. Then as per Phil Greeno does, I always wash with panel wipe (sometimes labelled as "pre-paint" to remove grease, silicon, finger print grease etc. Finally wipe with tack cloth seconds before painting. I use either Halfords plastic primer aerosol cans, or Klass Kote epoxy primer to get a really good grip onto epoxy glass. From then on handle the parts with gloves on to prevent finger print grease on the primer, sand back with 1200 or scotch grey pad, then again tack cloth immediately before painting.

One "gotcha" to avoid - many people measure out their paint and thinners using a medical syringe, and then get fish-eye on the model indicating silicon contamination despite scrupulously cleaning the model prior to painting. The contamination comes from the lubricant in the syringe so if you do use a syringe wash it out thoroughly first.

Washing composites with soapy water to remove grease, mould release etc can contaminate the model as much again, for example I live in a hard water area and after a water wash the model has lots of fine deposits of limescale on it, so nowadays I only use thinners, panel wipe etc.