POR-15 is not a rust converter, its a coating/encapsulator. If your going to let it set for months on end prior to paint consider using a phosphoric acid rust converter, "Ospho", navel jelly, etc...

these convert iron oxide(rust) into iron phosphate that coatable.

Then clean throughly, wire bruch, get all rust out then coat with POR-15. This stuff is as hard as a rock when set up, if this is inside a rocker panel or inside the vehicle thats probably a good location for this. If your talking about exterior panels with top coats of base & clear to go above this then generally its not recommended, its too slick & too hard. you can also consider an epoxy coating as long as its not in the sun (no UV) instead of POR-15.

If you dont do something like this method first, rust will reappear.

http://www.ospho.com/

Here is an excellent auto body site to search thru as well.

http://www.autobody101.com/forums/