I knew I had a dodgy floor that needed sorting but was expecting a little more metal around the edge that I could plug weld onto. But also I've found that it's unibody with the whole floor made of a complete single pressed sheet. If I wanted to try and make the repair (new front floor pans off ebay) look original then I was thinking of butt welding them in might be the only way to go. But does anyone know how you would you cut the shape out for new floor pans but leave a small enough gap to mig weld them in? Having never welded before but being lent a mig welder I can see me ending up with a mess. Youtube videos seem to be lapping them in.

You need to standardize the shapes you use. As an example, the last photo next to the scrapper I would cut back until I had a triangle. Get some stiff card and a marker and trace around the inside to get your template. The fun part is that it's now a case of trial and error. It basically is a bench exercise in that you keep filing until the shape fits.

You will need some files, a grinder/nibbler, and a hammer to adjust your edges.

Personally, unless it's a mint car, I'd patch with overlapping patches - especially if you've never MIG welded before: otherwise you'll be blowing holes all over the place. If you do try to let patches in with butt welds, avoid sharp corners because, for instance, the points of a triangle will be very difficult to weld effectively.

The things that will make a difference to your job are:
1. Excellent preparation: ensure the surfaces you are welding are as clean and solid as possible.
2. The quality of your MIG set: a cheap hobby set is much harder to use.
3. Work slowly: short tags of weld that don't put too much heat into the metal will really help.

But I would overlap patch. It will be easier, stronger, and likely last longer.

+1.
Additionally I would consider buying a joddler which will make it easier to produce a neat flush joint.

+2 for overlapping, also cut the areas out larger and square as it'll make joggling and welding easier.

Thanks for the advice guys - really useful especially as I'd never even heard of a joddler before. Although the car is nowhere near mint (more like the ratted / oily rag look) it's original to the point of everything hanging on by a thread. So the approach is to try and keep it as close to that as possible and not over-restore. Not everyone's cup of tea I know. But I think on balance - yes lapped joints are the way to go as they'll give me a bit more room for error.

If I was closer to where you're based Joe I'd be leaving it in your capable hands.

Me too!

Damn. Spent today cleaning up ready to cut out, but the rust has gone further up the foot well than I had hoped. Not going to be as straightforward just replacing the floorpans. I can buy re-manufactured floor pans but not the bit that goes vertically up the footwell. Think that reluctantly I might have to pass on attempting this job myself as I don't have the kit to make something profiled like this up. Any last ditch ideas before I bale out on it and take the pain of taking it to someone?

I'm not sure I understand what you mean. There looks to be a hole or two that you could easily patch.

For inspiration

http://www.alfa.me.uk/alfa-during.html

http://www.alfa.me.uk/crash.html

I've got my head around replacing the floorpan - I'm going to form a recess with a joggler and then lap the new panel in and plug weld it using new floor pans that I can get off ebay:



But I'm less sure about cutting out the panel that starts going up towards the engine bay (where your feet rest). It seems to be some sort of double skin construction plus the subframe obscures seeing much from the underneath. I guess I should just cut it out and see what I find. The photos don't really show it well, but it's profiled at the edge nearest the door to accommodate the wheel arch - and I'm going to struggle to start making up anything that has to be shaped. Looking at the state of the metal it looks like
most if not all of the full width of each pan will have to come out and up about 6-8 inches above.
It's by no means a mint car but it's a 1st year of the 1st series fulvia so don't want to butcher it too much. Especially as this will be my 1st attempt a welding (using a 2nd hand clarke 151TE).

wow - just looked at the crash reconstruction. My problems pale into insignificance. I think I should just get on with it and stop worrying.

I would say with the floor pan that is available you can fairly easily create patches that will sort the holes in the main floor panel (or just weld in the complete floor pan). For the kickboard part you could create patches, most of them will be flat metal so just buy enough sheet for that, it is only the part that curves towards the transmission tunnel that would need shaping and it looks like a reasonably simple curve that just needs something to bend a piece of metal around to make the required patch. Possibly depends how accessible the other side is as to how difficult it will be.

MRS - very impressed by the repair you did after Castle Combe!

If you're only patching, you don't need to worry about "butchering" the car: in fact some good patches would be better than wholesale panel replacement done badly.

A point Chris. Don't cut out all the corroded panels at once.

The shell can become distorted if too much metal is removed at once, it is easier to work if you have the opposite side intact to see how things should fit together.

Consider adding extra temporary bracing before cutting out panels to make it easier to keep the shell true whilst working on it.

Thanks again for the advice guys - I think I need to stop wimping about and bite the bullet and just get on with it. I'll aim do it in stages to avoid weakening it like you say Pete.