I t will depend on the condition of the paint on the bar now, if the paint is in goo nick all you need is a light rub ( enough to take all the shine off ). You dont need to prime just the finishing paint.
If you are going to use 2 pack ( you cant spray outside , you need a spray booth ) then 600 paper is good enough, spray and leave gun finish . If you are going to use acrylic use 800 paper, spray the colour on, cut back and polish .
If the paint is peeling or you have damage on the bar , repair the damaged part prime the the repaired part and finish as above, if you rub back to expose the plastic you will need plastic primer on the exposed plastic so the primer or finishing coat will stick, normal primer will not take to plastic well.