I agree with everybody that said to epoxy prime 1st, then apply the seam sealer. However I've used a 3M product called "Drip Check" to seal the rail gutters. It's thinner in consistency than regular type seam sealer, and can be applied to bare clean metal and then urethane primer over it. It flows out real nice and you can smooth it with your finger if required. I used that on my '54 Chevy Panel truck 14 years ago with no signs of cracking or rust thru. But in current practice everybody suggests epoxy prime 1st, then do all your body filler, seam sealer etc on top of the epoxy.