If you wash the panels down with a strong solution of TSP and water, that should strip off any surface contamination. Then clear rinse, and lightly sand with 120 grit paper (you're just trying to rough up the surface a little so that your primer can adhere.) Wipe them down with a tack cloth or a rag lightly dampened with mineral spirits/varsol/paint thinner, and let them dry for a couple of hours. Prime them with a high-adhesion oil-based primer, and top-coat with whatever you like. The primer and two coats of top-coat should pretty well mask the seams, but if they are still visible, you can always faux finish over them, or hide them with a stencil, or just go with your original idea of textured paint. Your local paint dealer should have texturing addivites for a couple of dollars that you just add to any latex paint.