What I did for surface prep was as follows:
1) peel off vinyl protective covering and wipe surface of the parts to be primed with Coleman lantern fuel (naphtha) to remove any adhesive from the vinyl. There are those that will say that there are oils in this fuel that leave a residue, but I have had absolutely no trouble with variprime peeling off of anything that I have primed as a result of any oily residue from its use. If it makes you feel any better, substitute lacquer thinner anywhere I am using Coleman lantern fuel. The same job is done, just a bit more expensive.
2) Scuff the surface with a Scotch Brite pad. As Capt. John said, this just gives the primer more surface area to bite into.
3) Wash the parts off with soap and water. This gets the majority of the dust from step 2 off the parts.
4) Wipe again with Coleman lantern fuel while wearing gloves to prevent getting oils from your hands on the parts.
5) Have at it with the Variprime.
All this sounds like an involved process, but it really isn't. I usually would plan a building session around priming and wait to do so until I had a pile of parts ready for primer.
Hope this helps.
Regards,