When you have painted it squirt some engine oil inside the doors until you see it running out of the drain holes, makes a bit of mess but it should stop it from rusting again.

I have been using new engine oil inside box sections, doors and sills for about 50 years and I can assure you it goes along way in preventing rust.

Waxoil is OK for flat surfaces but doesn't creep into the seams where most rust starts, before it sets, oil remains fluid and will go on creeping. If you do it once a year it will considerably prolong the life of the metal.

A film of oil will prevent both water and oxygen reaching the metal surface, without oxygen steel cannot rust.