when i did mine over a year ago, i just lightly sanded the plastic parts, then used flexi prime, scuffed that abit, then painted the parts using cans of touch up paint from supacheap, cant remember what grit the sandpaper was, but it was a fine grade.
after a year, mine looks really good still, the primer was about 20 bucks for a big can, and the paint was about six dollars for the smaller cans (they didnt have big cans in the colour that i wanted) and all this was bought at supacheap in the one aisle