The way i do repainting, is to take apart whatever that can come off. The rest that cant, just mask off with some tape, spray, and repeat. Since you are painting a G2 fig, its much easier since its so blocky
You can also sand the figure if u like, i did not, and it turned out OK still. But i do reccomend washing it in WARM water to clean off dirt/factory oil (Warm, not boiling).
I made the wrong choice of starting out with DOTM Megatron, which was a huge pain in the butt to paint since it has so many moving parts.
Oh, also, if the colour you are painting on is similar to the base plastic, i suggest using a silver basecoat 1st.