Depends what finish you want to go for. I've not worked with vinyl dye before, but the finishing principles should be the same as for raw plastic.. If you want a gloss mirror-like finish like the white g1 you shouldn't need a clear coat, you'll just need to sand and polish within an inch of your life.
If I was doing this, I'd use a fairly rough sandpaper to get the rubber off, then maybe a 400-600 grit to level the surface ready for painting. Apply the paint, cure it, then go at it with progressively fine grades down to at least 1200 grit.. maybe evn 2000. At this point you will have a smooth but not mirror finish, to get that you need some kind of polishing compound (3m finesse-it or perfect-it is good) and a buffing pad... and a truck full of elbow grease.
The main issue you'll have with this is the thickness of the plastic.. there isn't much of it.. You could easily go right through with all that sanding.
Samsung Galaxy S3 GT-I9300
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HTC Desire Z
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