Whats the best rust permanent stopper on say like a front steel door? Thanks

The problem with rust on doors is that it usually starts from the inside where water and debris collects in the front bottom corner of the door skin. The best way is to cut it out and weld in a patch. Otherwise, you need to really clean out the inside of the door (sand, leaves, loose rust) and apply a rust convertor made by loctite or rustoleum, then apply a protective coating on top (POR-15 or some paint), but this will only buy you some time; it's not a permanent repair.

If the rust is under control to start with... or after repair a waxy rust preventer helps a lot to prevent rust in closed areas like inside door bottoms. Get it completely clean and dry then apply the waxy oil like the Eastwood stuff until the bottom part of the door inside is well covered and it runs out the water drains. It sets up overnight. It will also smell for a few days but does go away.

http://www.eastwood.com/eastwood-heavy-duty-anti-rust-in-amber.html

I use fluid film

a yearly application of hydraulic/linseed oil mix works very well. this is used around here a bunch to counter act the effects of road salt/brine water used in the winter.

It would be an interesting experiment to buy a door from a junkyard that shows rust stains at the bottom and then soak the bottom of the door in EvapoRust for several days.

EvapoRust claims its fluid does not harm paint, plastic or rubber.

If the door bottom rust could be mostly removed then Rustoleum brand rust converting primer could be sprayed in the crevices prior to painting. I just used this primer on a 33 year old Amana air conditioner condensing fan circular wire grille and it seemed to do a reasonable job, but took longer to dry than the label said. The dried color was tan, not the Loctite Extend like black.

it depends on if it's older vintage or late model sheet metal and a LONG story about rust problems from a autobody tech that has to repair it.

if it's vintage the area between the panels is open. you can apply a seeping rustproofing like the eastwood stuff listed above. this type of rustproofing can flow into the small spaces between the panels where regular paint can't get to. VW and a lot of European cars uses tis type of rust prevention with excellent results.

on most modern cars the 2 panels have glue or some type of sealant between the panels when they are made. this prevents any type of rustproofing from creeping to prevent rust. the only problem with this method is there usually isn't any sealant on the metal that gets rolled over the other edge. now they dip the part in E-coat primer. because the sealant is between the 2 panels the primer can't get between the 2 panels in the inner seam. they don't apply any seam sealer before painting. because the edge of the panel folded over has bare metal exposed moisture can seep into the flange causing rust. there is almost nothing you can do to prevent rust from happening in this situation. you can apply sealer to the inner joint to prevent exposure to moisture. this will help some but once rust starts you can never stop it.
you see this problem on almost every new truck and a lot of cars from the mid 90's to present.
some manufacturers have started applying sealant to this seam with good results some manufacturers have started using more sealant between panels when they are formed with good results also.
after manufacturers partially solved the rusting seam problem they created another one with there extensive uses of foam between panels for sound control. there is no solution for foams absorbing water causing rust.