that looks like just surface for now, i would sand it down and put some rust stop on it

 

Get a little POR15 kit, and follow the instructions. It will do really well for you.

 

COS - That rust is most likely creeping from the inside of the door. Before you start, make sure the drain holes in the bottom of the door are clear, flush that out as well as you can, land let it dry well. Once you've done your POR-15 treatment, flood the inside/bottom inside with rust-proofing, Cosmoline, or something similar.

 

Do the POR15 thing. Also, remove the inside door panel and coat the bottom seam of the door from that side too.

 

I went to Duncan's dog house the other day to swap out his dog bed, and discovered yellowjackets flying in and out. Here's what I saw when I looked inside.



This is what it looked like a couple hours later when the sun was down.



No more yellowjackets.

 

Good point about the insides. Will the $25 kit be enough for four doors or would that require the bigger one, or would it make more sense to do something like POR15 on the outside and a different product like cosmoline on the inside?

 

Looks like a 4 oz can in that kit.
I don't know, seems like enough but.....
I blasted and two coated an entire trailer hitch with some leftover from a 4oz can but I don't really know how much the doors will take.
The paint is very thin and does go a good ways.

EDIT:
From website:POR-15 application on non porous surfaces (most metals):
Approximate coverage:

Pint = 50 sq ft (Single Coat)
Quart = 100 sq ft (Single Coat)
Gallon = 400 sq ft (Single Coat)

So, per the maker it will cover 12.5 ft^2
Seems like enough if all doors only have the amount of rust shown in the pics.

 

Thanks for that, that does seem like it should be plenty.

 

That kit is plenty for what you're doing. It will also coat a driveway pretty well. (Don't ask)