I'm not sure if you mean the front corners or the back corners, but I sealed both of mine with POR-15. Mine's a Colorado truck but it had rusted through wherever crud had accumulated. I wire-brushed it and painted on POR-15; that was 5 or 6 years ago and it still looks great, no rust, although I occasionally find condensation. For the bigger holes I used POR-Putty and fiberglass cloth. Worked great.

Good luck.

Don't read the directions, that'd be too easy.