Can I install each piece separately, or do they need to go on at the same time?

Each piece installs independently. You can finish and mount them one at a time or all at once — install order won't affect fit or alignment. Just dry-fit everything first to verify gaps before committing to fasteners or adhesive.

Do I need seam tape or gap filler between the pieces?

Depends on your prep. If gaps are tight and consistent, paint over them. Wider seams benefit from automotive seam sealer applied before primer — check your gaps during dry-fit to decide.

What's the best way to paint FRP — can I spray all six pieces at once or paint separately?

Painting them separately lets you cure each piece before handling and stacking. If you spray them all together, use spacers to maintain air flow and prevent adhesion during cure. Either method works — it's about your workspace and drying time.