The product you are using is an excellent choice. The biggest thing is getting all the old seam sealer out, then use an etching or epoxy primer in the drip rail area. Once that has cured, scuff it up with a Red Scotchbrite Pad for adhesion, tape it off to avoid spillage and fill the rain gutters full.
That two part seam sealer cures quickly so it needs to be applied quickly and definitely not to be done in the sun.
Paint the seam sealer in the same manor, with the same paint used on the vehicle. You won't be doing it again.
Better to return one tube than run out while doing the work. | 
