Leaking roof

[why me]

hi guys my mate has had the same problem with rear seam in roof leaking out of waranty so me being a mechanic 4 20yrs thought the best cure would be slightly open joint with blade scraper and use brushable seamsealer from local bodyshop and force in to joint and tap back together when dried painted over with white hamarite all looked good gues who turned up today with a bucket in van GOOD JOB FORD DONT BUILD HOUSES can any one let me know what sealer is best to use as the seam sealer i used oveusly dosent like the vibration/movement on the joint (need to be 100% water proof as hes a plastera and ready mix plaster with water doesent last long ha ha )