Huh?

I use an epoxy primer in any areas that get sealed up, grind away only small areas for spot welds, treat them with weld through primer.  Once the car is painted, then I use a rust preventative treatment, like Rust Check, which will crawl into the seams and coat any areas that got burned away with the spot welds..  Some info from Autobody101 forum..

Etch primer uses a chemical (acid) to adhere to the metal.
Etch primer cures faster and is the choice of collision shops.
With etch primer you need to apply your filler first directly to the metal.

Epoxy uses a mechanical bond and requires the metal to be media blasted or sanded prior to application.
Epoxy has anti-corrosion properties and some has UV inhibitors as well.
Filler can be applied directly over epoxy primer and more epoxy applied over the filler to completely encase it in epoxy.
Epoxy also provides some build so it can be used exclusively without any other primer/surfacer.
Epoxy can be reduced 50% and used as a sealer before base coat.

Weld-through primer is just that - it is a base coating designed to be welded through and nothing else, and has no etch capability.  Painting with a proper etch primer and then spot removing where required for welding is a better bet.  I've never really gotten on very well with welding through it anyway, the high zinc content spits and fumes off in a toxic manner like welding galvanised steel.

Another tool I've found very useful in cutting out sheet metal is the Dremel.

You're in the Carolinas. It will start rusting before you can get all of the paint off. Either prime it as you go with a weld-through primer, or use the epoxy primer and just be prepared to grind back the areas where you need to weld later on. Not that big a deal.

Oh I go to Raleigh/Durham pretty often. I'm actually closer to Sanford.

Yeah that might be a good idea. I'd be willing to just grind down the area I'm going to patch later to prevent alot of rust. 

I am in Durham... not exactly close but not too far away either.

What you might want to do is determine where your spot welds are going to be, mask those small areas, and prime the remainder of the floor before spot or plug welding.  That would limit any flash or surface rust to very small areas that you can easily clean when you return from deployment.

Well I also am in central North Carolina. Fort Bragg to be exact. How close are you I could use a mini buddy haha. Well I already stripped the floor pan and under. So I might just epoxy primer it before I leave

As you said, how long it takes for rust to surface is situational.  I am in central NC.  We can have awful bouts of condensing humidity that pop up without warning.  In my case... flash rust could happen in a day... or take months.  I never know.

How much of the car are you planning on stripping?  What will you be stripping the paint with?

Nice I think epoxy primer will fit the bill. I know it's pretty situational but how long can I leave it bare metal before I have to worry about surface rust again. I'm in the army and will actually be deploying in about 3 weeks. I know I picked a good time to start haha. It's only looking like a 4 month deployment. That being said my plan before I deploy is to get as much stripped and ready to be welded in before I leave. However I plan on doing the welding when I return.

I've got the outer sill off on one side and the rust doesn't seem to be that bad. I'll send some pictures for recommendation on what peices I should get but I'm leaning towards. 

Inner sills
Out sills
A frame and step
Valance

Everything else can be pieces from scrap 18g metal.

Sissy primer?  I've never heard that term before.

You could prime with rattle can sandable if you want.  However, if you are putting the effort into the repairs, don't want to buy primer twice, don't want to strip the panel multiple times, and want a real protective coating... buy an epoxy primer that will be compatible with the top coat you are planning on.  The epoxy is an excellent moisture barrier for panels/repairs that are done piecemeal and can be rough sanded for filler application.  You don't have to mix big batches if you measure carefully and you can use a small panel gun if you are only spraying a couple of square feet at a time.

I still have to hit "reply" to see any posts after the first one.  I am not using a cell phone.  I am running Chrome on a Dell 64 bit, Win10 laptop.

Mini mania is not set up for phone use very well. 
Another question I just thought of. Since this is going to be taking me some time to complete I don't want new pieces to be exposed. Any recommendation on a sissy primer after the pieces are welded in. I'm not going to be painting it for quite some time. Also so I can primer the inside of the inner sill

I am having the same problem seeing the posts on my I phone, no problem on a PC.

Thanks for the replies gents. I watched a video on YouTube called classic mini floor and sill replacement. It's over an hour long. I'm tracking how the construction is made now. Pulled out the welder and practiced a bit cause it's been a while, still looking fine though. I'm going to get the outer sill of and definitely replace that. I think I can spot weld those holes in the step abd patch where the seem is and also all the other holes.

+1 to using regular sills and not oversills. Even cut into smaller sections, they will fit better.

First... I can ONLY see the first post by the OP until I hit "reply".  I cannot scroll down beyond the first post until I start my own reply.  Is anyone else experiencing this?

Do not use the oversills even for the mode you suggest.  They are immediately recognizable as oversills by anyone familiar with Minis.  So what?  Even though you are suggesting using them in a benign manner, it will suggest to others that you have taken shortcuts.  Maintain the value of your car by using regular sills.

You asked about welding in a stiffening bar (across the door opening) before replacing the sills and doorstep.  I would.  If you remove both the sill and doorstep without adding the stiffening bar there is a chance the door aperature will distort some during your repairs.  A simple, small piece of angle iron across the bottom of the door opening is all you need.

No need to spot weld. You'll probably will be making small  pieces in that area. Same with the door step, but that will be determined by removing outer sill. You'll then see how the mini in constructed with the outer and inner sill.

Hey thanks for the response.

So my though with the oversills was to use them just as a larger patch, but i still planned on butting them up with the current floor pan. Yeah i thought about just placing it on its side and taking care of the rust but ill be ok welding on my back. The welder i have currently is a Mig gas welder similar to the Lincoln, so im good there. I also have air tools/grinders/ and hole punches.

I think i will start of by removing the outer sills and see what is inside. My biggest concern is the bottom seal and the A frame door skin. The second concern is the door steps. That's why i was thinking of getting a door step repair panel.

Before removing outer sill should i spot weld in a support in the door frame?

Any more advice is appreciative.

Welcome to the world of mini ownership. First, do not buy the oversills. That is for quick and dirty repair to pass MOT. What you will end up with is a worse condition than before. Any time you lap metal, it rusts worse.Buy heritage sills. You want to butt weld the pieces together. I looked at your video. You have an average or better than for that year. Most late models rust worse. Start by taking off the outer sills. this way you can make patch panels for the floors and inner sills.

Too bad you removed the rear subframe, because you could lift the mini up and place it on it side. You place a chair or something sturdy where the roof and door meet. This way you are standing while welding or grinding. Not laying on your back. ( drain the fluids from engine).

Get yourself a mig (Lincoln) with gas from Home Depot. A paddle switch type angle grinder and a hand hole punch. The hole punch is for the outer sills. 

I would leave the door step as the aftermarket replacement bend are not as sharp as the originals. Just make replacement pieces. You will know more after you remove the outer sill and take a look from below.

If you can fabricate a stand using the rear subframe bolt holes the support the rear and put the mini on its side ,you'll be much happier.

Don't bite off more than you can chew.

Start with the floor. Work one side at a time. This way you can see how things go back together. Remove the old patches and outer sill. Make patches large enough to get rid of rust and cover the holes. You will need to buy 18ga sheet metal.

Afternoon,
I've got a 1992 1275 Mini that I'm looking at taking care of the rust problem once and for all. I've removed all the undercoating on the floor pans and under the car and start exposing some of the rusty problem areas. I know how to weld (not great) and have a welder (not great). I have welder body panels before but small patch jobs.

Video of the car/rust
https://youtu.be/m6W5bBDayvI

I am looking for advice on what panels I need to buy to weld in.
Here is a list of what I think I need
Classic Mini Step Door Jam All Models 34.5 Long Part No: MS16
Classic Mini Oversill 9 Standard Mkiii & Later
Aftermarket Classic Mini Door Step & Lower Panel Classic Mini Inner Sill Panel
Aftermarket Classic Mini A Post & Door Step Repair
Classic Mini Valance Rear With Metric Lamp Fixing Nuts
Classic Mini Rear Closing Valance Panel

I think this is the majority of the panels I need. If the video is bad to let me know and ill take it with an actual camera, not my phone. Any advice is helpful. I am located at Fort Bragg, NC if anyone it close to here and wants to help work on my mini, come on down I could use some advice/help.

URL: https://youtu.be/m6W5bBDayvI