Smooth out the seam using either Bondo, Duraglass, or other body fillers. Body fillers can be purchased at auto supply stores (AutoZone, Walmart, etc.).
Sand down the body filler using the following grits in stages: 80 to 180 to 320. The values do not have to be exact, meaning 220 can be used in place of 180.
For best result use a sanding block to make sure there are no high or low spots.
Follow with primer and paint.