I use Plexus to maintain and clean. Meguire's Plasti-X for more serious cleaning, and for restoration work, 3M's Headlight Lens Restoration System Kit (Part #2516). Below image shows a Vista dome that was partially restored for comparison. That dome had been in service for five years.
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"All lightbars require care for the lenses or domes. Consult with your lightbar manufacturer for the recommended procedures. Lenses should be cleaned frequently using mild soap and water. Avoid brushes, high-pressure spraying, and cleaning in sunlight. Plexus® is a plastic polish product available from Federal Signal distributors available at most motorcycle dealers. Developed to clean aircraft windshields, it is an excellent choice for routine cleaning. Plexus leaves a protective film on lenses—much like furniture polish protects wood—and keeps bugs from sticking.
Plastic lenses and domes will oxidize over time and eventually require replacement. As the cost to completely replace domes or lenses runs $100 or more, it can be far more economical to restore them. I have had excellent results with the 3M™ Headlight Lens Restoration System Kit (Part #2516). This complete solution restores headlight, spotlight, and lightbar lenses and domes to like-new condition.
The kit pays for itself in only a few uses, saving hundreds of dollars in replacement costs per vehicle. Restoration could double the normal service life for the lenses, keeping them effective for the vehicle’s full lifecycle. Be aware that manufacturers generally recommend against routine abrasive polishing of the lenses. But, at the point that they are weathered enough for replacement, the restoration process can only help."
Excerpt and image from my "Maintaining Warning Sytems" article printed in Police Fleet Manager Magazine July 2009. Full article available here: http://www.hendonpub.com/resources/articlearchive/details.aspx?ID=207674