If you have a typical lap-weld with a raised bead down the seam, for strength, then you may not be happy with any of the Bondos and epoxies. This would be a panel that is visible and not something on the floor that you cover with a mat. You fill-in along each side of the bead and sand smooth and uniform but it will still be obvious after painting (light reflection on a raised surface). If you don't want evidence of where you have worked I would consider covering it with some type of interior fabric or a pre-formed covering.
