The drain holes get trash in them and don't let the rain/snow/salt out. I check mine regularly and have found water in the doors a time or two. Never understood why the weather seal had to be right on the drain holes. Maybe wire wheel the rust off and bedline it?
POR-15, buy the small can for ~$11 on Amazon. Just apply couple thin coats. Do your inner back wheel wells also. Small can goes along ways and once the can is open and then try to reseal, it never reopen's the 2nd time. End up throwing it away. If exposed to lots of sun, your suppose to put paint over it.
Appreciate this thread - going to make me look under the stripping on the door edges of my 2012 P4X, and the rear storage door is rusting, so ordered a can of the POR-15. Just need it to last another year or two.
Couple times a year I squirt water from hose in there and flush it out. Also, get a can of fluid film and squit it in all the drain holes in all the doors and in your bed.....there are grommets that you can pop out. Dump half a can on each side to coat the rear quarters. Never rust again!