When I decide to invest the time, I’ll garage the car for three to four weeks straight after a thorough ¾ step detail. Just after the detail, P21s goes on the car. Wait 4 to 5 days, and apply another coat. P21s completely ‘cures/hardens’ after several days. Repeating this step after another 4 to 5 days. I don’t use any sealants though, just P21s. I did go to a 4th and 5th coat on a former black P-Car over one winter and thought it even better than just 3 coats. (I’ll jack the car, pull the rims and do the front and backside of the rims at the same time too.)
Solid black and white cars seem to really look wet AND have depth to the paint that I (opinion) can’t get with any other technique. On my black metallic (red on the Merc & blue on the Honda) I love how it seems to amplify the ‘glitter’ of the metallic.
Hope this helps.