Are you spraying the van roof or the pop top. On most conversions there is a wooden frame screwed to the van roof from underneath. You have to remove the headlining to get to the screws.
On Autosleepers, the pop top is fastened to the frame with screws from the top. These invariably brake off and have to be re-drilled.
I only remove roofs if I'm doing a complete resto or scrapping. Panky's suggestion of masking is easier. Daisy has just had a respray and we lifted the top and masked it all