While I had my right rear off, I scraped away all the undercoating. What I found was a two edged sword. Where it covered properly, everything was all right underneath. Other places, mainly due it appears, to being applied after assembly the undercoating trapped water and dirt and held it against the metal causing rust and pinholes.
After sandblasting, I painted the entire inner surface and the portion that is the faying surface to the body tub with POR 15. To protect it from rock chips I painted over that with clear product used on lower rocker panels.
Maybe this was over kill, but it gives me peace of mind, as I drive all year in the Northwest, which is not the driest place in the country.
I also spray the tire sling area with LPS 3 which is easy to use for touching up.