[Moneyguy1]

"Extend", or any other rust converters have to be sealed because they are water soluable. I have (since I used to live in the rust belt and did a lot of restorations) had good luck by using epoxy over the rust converter, then primed and painted. You can add a flexing agent to the epoxy to make it les brittle and subject to cracking. Look up West Systems. As an aside....Lousy design; creating a place for water to collect that cannot easily drain away or be cleaned. Might consider filling the cavities (after doing the sealing/preservation thing) with expandable foam which in turn would it self require sealing. Good luck. You have your work cut out for you.