As I mentioned, I would not want to break up a matched set, but I did some research in the "other forum" and came up with a range from $37 for "pretty solid/no filler" to $75-150 "reskinned", all the way up to $281 "fully refurbished" for a single door...

Note the other forum is http only, but this forum is auto-prefixing all the links with https: so you'll get a "secure page" warning initially when you click on the below links, but just edit the URL to remove the "s" (so it reads //... only), and then you can see the pages...

1. "pretty solid" $37 //www.theminiforum.co.uk/forums/topic/324247-doors-mirrors-lights-and-more/?hl=door+for+sale
2. "no filler" $37 //www.theminiforum.co.uk/forums/topic/286589-drivers-door/?hl=door+for+sale#entry3116300
3. "rust free" $43 //www.theminiforum.co.uk/forums/topic/293102-drivers-door-wiper-motor-and-other-bits-and-pieces/?hl=door+for+sale
4. "virtually rot free" with glass and winder $56 //www.theminiforum.co.uk/forums/topic/290105-drivers-door-arches-bumpers-and-mk1-bonnet/?hl=door+for+sale#entry3140884
5. "fair to good" $62 //www.theminiforum.co.uk/forums/topic/274972-excellent-passengers-door-bare-l70/?hl=door+for+sale
6. "pretty good nick" $62 //www.theminiforum.co.uk/forums/topic/321189-drivers-side-door/?hl=door+for+sale
7. reskinned $75 //www.theminiforum.co.uk/forums/topic/265936-nside-mini-door-l60-will-be-at-castle-combe/?hl=door+for+sale
8. reskinned $93 //www.theminiforum.co.uk/forums/topic/290396-drivers-door-mk3-onwards-bonnet-and-clubman1100-special-bumpers/?hl=door+for+sale
9. with glass $95 //www.theminiforum.co.uk/forums/topic/249395-2-pair-of-doors/?hl=door+for+sale
10. "does not need re skin" $125 //www.theminiforum.co.uk/forums/topic/304179-passenger-side-door-for-1984-mini-in-the-south-east/?hl=door+for+sale
11. reskinned $125 //www.theminiforum.co.uk/forums/topic/272349-spare-parts-for-sale/?hl=door+for+sale
12. reskinned $150 //www.theminiforum.co.uk/forums/topic/238587-reskinned-drivers-door/?hl=door+for+sale
13. reskinned $150 //www.theminiforum.co.uk/forums/topic/271354-2-re-skinned-doors/?hl=door+for+sale
14. "totally re skinned and rust free" $150 //www.theminiforum.co.uk/forums/topic/261703-re-skinned-passenger-door-l120/?hl=door+for+sale
15. "good one" $187 (includes UK shipping) //www.theminiforum.co.uk/forums/topic/331748-mpi-passenger-door/?hl=door+for+sale
16. fully refurbished $281 //www.theminiforum.co.uk/forums/topic/288646-fully-refurbished-doors/ ($218 with exchange)

I should note that some of these prices go back several years, so there is likely some inflation to take into account.  Let's guesstimate a 5% markup.

And none of these include shipping from the UK to the US.  I don't have a shipping quote for a door from over there, but I do have a one for a door skin, plus the boot repair panel that I mentioned earlier.  That was seventy Pound Sterling (~$87 because it has to be by courier), so it is probably reasonable to believe that a full door's shipping cost would be more (since it is not only bigger, but heavier), maybe around $100-$125?

Whats a good price for a good door or a matched pair from 99 Mini 40? Steve (CTR)

I wouldn't want to break up your set Willie_B and Dan's suggestion of doing all the fold work myself and just temporarily using pop-rivets (until a body shop can make the proper welds) could keep my overall costs pretty low if I'm only buying a single new skin.

Unless Dr Mini has a single door at a good price (he said he'd check what he has), I'm beginning to think I'm full circle on this being a DIY/WaLH (With a Little Help), but looking at a much better end results due to all the great advice everyone has offered!

There is no weight savings. The fiberglass skin has to be thicker than metal to retain it's shape so the weight is still there. The door frame/inner part is still the same metal so no problem supporting the window and winder. Not willing to sell just one door though. The set would be $600. I don't live far from Dr Mini so we could work out transportation.

"Fiberglass skin" Willie_B?  Wow, I didn't even know that was an option!  I wonder if that would support the weight of my roll-up windows...?

Finally took photos of the rusting door - see here (that tape is just to try to keep water out of the PO's peeling off Bondo area)

I also took a series of photos of the underside of the door as well - here.  Except for the front corner (left side of pics), it looks like the Lip might be intact/solid enough to re-skin without having to weld extra bits in. 

But just fitting a replacement door from Willie_B or Dr Mini would certainly be quicker and possibly more cost effective compared to the purchase, shipping & painting cost of a new skin.

What would one door cost Willie_B?  And what would it weigh? (to calculate my shipping costs from GA to NYC)

It might just come down whatever the least expensive option is since I have a rusted section in the boot that I also need to get repaired - see here - and I know that will definitely require cutting & welding (which I don't have the equipment or wherewithal to execute on), so I'll have to pay a bodyshop for that.

Hmmm... lots to think about!

I have a set of fiberglass skinned doors available.

Great, Thank you Dr. Mini.

Looking forward to hearing back from you.

I will check and see if I have anything. As far as me realigning doors, no I don't do stuff like that.

That certainly could be a good option Dr. Mini.

(And I could meet you at a friend's driveway in Forest Hills/Queens so that you wouldn't have to come through Manhattan on your way to L.I.)

It's the right-hand side (when sitting in the car facing forward) that is the really bad one.

(The left side has some minor bubbling, but nothing as bad as the right side where there's even bondo coming away from the skin)

What cost were you thinking for the replacement door?

And would you want to take the old frame in some sort of partial exchange?  (Do you do your own door re-skinning/restorations? ...I'm obviously reading into the "Dr. Mini" moniker with that question!)

If so, I should strip all the internals on my door ahead of time for you so as not to hang you up timewise.

Which door are you talking about, left or right? Perhaps a good used one would be a better alternative to re-skinning. Let me know as I will be travelling to Long Island soon and could easily drop one off with you.

Oh, that is a good recommendation too malsal!

Where the skin and the inner frame meet, it will form a tiny valley in-between to hold any water that gets past the seals at the bottom of the wind-up windows - definitely not good.

I'm guessing perhaps a bead of seam sealer along the meeting point couldn't hurt either, before coating everything, just to fill the valley and create a "run-off slope"...?

re: POR-15, I'm actually going to be trying a product called "Rust Destroyer" https://rust007.com/destroyer.html when I grind back a bunch of niggling bubbling paint spots this summer.  It seems similar to the Buzzweld Rust-Prime others in the UK are looking at but doesn't seem to have the same restrictions on what surfaces it can be applied to.

 

After doing a lot of research on other rust converter options (including encapsulators like POR15), I chose Rust Destroyer, figuring they seem to stand behind their product, because it is "warranted to stop existing rust and prevent new rust for a period of 5 years...limited solely to the cost of product replacement.  Any and all labor costs and/or consequential damages are hereby excluded".

 

I haven't used it yet so I can't offer a proper review, but I did correspond with the company to try to figure out prep times - and the answer below came from a "Walter Landzettel" at Advanced Protective Products:

Thanks for the reminder of the option for just replacing the lower section repair panel too malsal. 

Unfortunately the PO did some Bondo repairs to the upper-middle-front-edge of the door, way above the 4 inches high mark, and the skin underneath the Bondo is what is rusting and "bulging" the Bondo outward off the skin itself...

...so I'm in for a penny, in for a pound on a full skin replacement (but not for a full new door - at easily over $1000 shipped to NYC, that's way too expensive!) 

Malsal's suggestion of the 4" bottom panel is a possibility, though it would require welding along the seam, followed by filler for a smooth result. If you do use it, seal that seam with POR-15 or equal. Otherwise you'll eventually have a rust line there.
If you opt for something welded, prime the new metal and any bare metal with weld-thru primer. Any "real" paint will inhibit the welding.

Whatever you decide to do make sure you seal the inside bottom sections of the door maybe with an encapsulating product like POR 15.
You need to especially seal where the skin meets the door frame as that is a water trap and where the rust will start again. 
They do make lower section repair panels about 4 inches high so you do not need to replace the whole skin.

Thanks for the quick replies kermy, Dan, SCUM & dklawson!

I hadn't thought about the dissimilar metal / galvanic rusting issue - living in NYC, I definitely have salt air and drive it in the wet AND have salty roads, so that is an important CON for going Aluminium.  I'm assuming that the salt and the two metals basically make a battery, but I wouldn't have thought of that myself!

Thanks for the suggestion of pop rivets to fix the skins in place until I get an auto body shop to have the welds done properly Dan - again, not something I would have come up with on my own, but a tactic that might make this a doable DYI/WALH job.  (Do-It-Yourself/With-A-Little-Help)

Good point dklawson: I do not know yet whether I'll need the inner door frame repair panels or not yet - the bottoms of the doors are still sound, so I don't think I need the door bottom repair panel, but I don't know yet what I have to "fold over onto" yet.  I'll need to remove the skins to assess that.

Oh, and she's NOT show car - I just driver her!

--------------------------------------------------------------

A side note / follow-up:

I have a 1991 Mainstream Mini Cooper, and as such the doors do not have the crashbar which was added in 1996. But since I live and drive my Mini in New York City, this might be a sensible addition!

At 1:51-2:17 in this video...

https://www.youtube.com/watch?v=Ryh8Q7EwduI

...you can see the late model crash bar simply being screwed into the inner frame above the roll-up window channel.

That, in my estimation, would make it an easy add-on to an older door while the door skin is off: Beyond possibly drilling the two holes in the front & rear of the inner frame, at most one might want to weld in a strengthening plate on each side of the frame, front and back... but other than that it seems almost a "drop in" modification.

I reached out to British Motor Heritage to ask if I could buy two door side crashbars (and screws) as a separate purchase and they replied:


So it sounds like they might be willing to come up with an ad hoc price and simply pull two crashbars off the assembly line...

As long as it is not crazy-expensive, do folks see this is a worthy "upgrade" to pursue while I have the skins off anyway?

+1 for steel and for Scum's observation about the single tack weld (or braze).  Most of the skin is just folded over w/o welding.

Start by removing the old Bondo and other repair materials.  Determine how bad it really is.  If the skin is damaged bad enough to warrant replacing chances are the door frame probably needs bottom sections repaired (read that as welding required).  If on the other hand the damage is minimal and this is a driver (as opposed to a show car) consider a POR-15 and fiberglass repair.   

Steel door panels are just crimped into place with one small section at the top welded.  Mini restoration books show you how to do it.  Steel panels are a lot less costly than alloy too!

Depending on the aluminum and your location, it may rot out faster than steel. If you have salt air or drive it in the wet or have salty roads, the steel and aluminum can cause a galvanic action that results in the aluminum rotting away.

I would opt for steel, fit the panels and use pop rivets to fix them in place until you can get to an auto body shop to have the welds done properly. Once welded, they can replace the rivets with welds. I don't weld, so that's how I did my door sill.

I've not done it but I believe steel skins are just folded over around the edges of the door frame. There's no welding involved.

Need to reskin the passenger door on my 1991 Mainstream Cooper due to bottom rust and PO Bondo layer.

Considering going with Aluminium because A. they are touted to not rust and B. since I don't weld and this article talks about epoxy'ing the skins on, I thought I could still DIY it.

Can anyone recommend going with aluminium Door Skins over Heritage steel?