Don't use cell paint,most vans are painted in 2/3 pack paint.
The cell paint can cause a reaction with the existing paint,
Use exterior gloss paint,and apply it with a gloss roller.
Don't use that none drip type liquid gloss is the best,several coats will give you a nice finish.
Spend some time on preparing the roof prior to painting,flat the whole area by hand or orbital sander,the grade of paper used is your choice.
1200 fine grade will do.
paul
April 2012