Bear in mind that the bolts each screw into a captive nut, held within the roof structure. That captive nut can move around a little, so sealant on the bolt threads is not enough. You have to put sealant all around the hole in the roof skin, so that it goes down and around the captive nut as well as in the threads. There is much greater scope for water to leak around the sides of the captive nut, than down the threads.
Also use the washers provided, or the ones that come with the van that sit under the bolt head.
Pete
Also use the washers provided, or the ones that come with the van that sit under the bolt head.
Pete