My other Early A ---65 Dart post

Heres the driver side quarter panel where it meets the door. Small hole on the outside but it looks like it rusted from the inside out. With the rear window and regulator assembly removed Ive been spraying the rusty area with ospho and since it thin like water its just running down and escaping through the drain hole that goes into the rocker panel and then out onto the ground. I used my cordless drill to open up the drain hole just enough to get a rubber plug into it. Now when I spray it with ospho it puddles up inside and can keep working at the rust instead of just leaking all over the ground. Ive got an old windshield washer pump and jug to fill with the ospho and was planning to use a small hose with multiple small holes to stuff up in around the wheelwells and just let it spray for a while.