You need to use a compressor to blow out all the excess water out of the seam first then use a hot air gun or carefully use a blow torch to thoroughly dry it out first or the sealant wont take. Wipe with meths, (should be in every good transit owners drinks cabinet,) and then mask 1/2" either side of the seam and liberally apply sealant to and under the seam, rubbing it in with your finger. Apply some more sealant between the masking and use a decorators scrapper or credit/loyalty type card to spread the sealant neatly along the seam in one long movement, side to side. Remove the masking tape in one clean peel and you should have a good seal that looks very neat. Use a good quality UV stable outdoor sealant. It's a piece of p!$$ to do.
