(91 Chevy K1500) Painting small strip behind top portion of door molding where the top of the door meets the roof

Hi everyone,
I'm hoping someone can offer some feedback on the following problem.
I have a 91 Chevy K1500 Pickup and there is a small strip behind the
top portion of the door molding (where the top portion of the door meets the roof) that I cannot seem to get paint to stay on. I've brushed on factory touch-up paint there and it always seems to peel off. It's just a small 1/4" wide strip, it' not on the actual roof, but it' where the top portion of the door meets the roof, behind the top portion of the removable door molding.
The surface has a smooth gray zinc or galvanized coating, and a few small light rust spots that I can sand off. I'm not sure if the peeling is because I have not been using the right paint or if it's because I have not prepared the surface properly.
Perhaps I should sand both the mild rust spots and the zinc coating completely off, and then prime and paint, I'm not sure.
I don't care how it looks, as it's not really seen unless you climb in the bed of the truck and look down between the top edge of the doors and the roof, or if you open the door and take the molding off behind the top edge of the door.
I have the following questions, please keep in mind that I don't care how it looks, I just want to protect the bare metal and prevent rusting...
1. what is the best way to prepare the surface so that the paint will adhere well and last, as opposed to peeling up in a year as it's been doing ?
2. Can you recommend an exact type and brand of primer and paint that I can use so that I know that they are compatible and that they will adhere well and last ?
3. Can I just brush the primer and paint on with a small brush, or is necessary to spray it on to get good adhesion and prevent peeling ?
I would prefer to brush it on since this would be so much easier, I would not have to mask anything off and won't have to worry about getting any paint on the cab interior, however, If I have to spray to do it right then that' s what I will do.
4. Since looks are not an issue, what other options do I have besides paint ? I thought of perhaps epoxy, but whatever is applied will have to resist deterioration from the sun and hold up to vehicle washing.
I have seen something used on vehicle body seams that appears or looks to be like an extremely tough bead of caulk, but much harder and tougher than standard caulk. What is this called and where can I purchase it ? Would this tough caulk be a good solution ? If I can just smear a coating over the area perhaps it would protect the surface and prevent further rusting.
I would appreciate any help or feedback.
Thanks John
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John may you can try some etching spray or BullDog adhesion promoter..I think I got it at Walmart.Or maybe you can get the brush on rust converter,it may stick to that. Bobby
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