Do I need to paint each piece separately or can I assemble first?

Paint each piece individually before installation. This ensures even coverage on all edges, prevents paint runs in tight seams, and lets you catch any imperfections before assembly. Once mounted, some areas become difficult to access cleanly.

Do I need a gap filler or seam tape where the pieces meet?

No gap filler is required. FRP panel edges are designed to mate cleanly when aligned properly. Use a small amount of flexible sealant along seams after installation if you want additional weatherproofing—standard automotive silicone works well for this purpose.

What should I use to prep the primer surface before painting?

Sand the entire surface with appropriate grit to dull the primer and improve paint adhesion. Wipe down with a tack cloth to remove dust, then apply your chosen primer or base coat per the paint manufacturer's instructions. Proper surface prep is what separates a clean finish from a rough one.