I knew I had a few rust issues simmering under the surface, I had done a couple of temporary fixes to buy myself some time. With the mot looming in November I had a couple of weeks in September put aside to sort it ready for winter. Then I spent one of those doing a clutch on the mrs' car and realised I probably wasn't going to get it done in time so concentrated on getting my blue 316i compact back on the road to use through winter. I soon realised it probably wasn't going to survive long term and was so slow I hated it. Fine as an occasional runabout, not as an only car. Which is where my silver 318ti came in. As winter dragged on and on I managed to get a few bits done to get it looking and handling as I wanted inbetween the weather knowing I wasn't really wasting any time I could have spent on the m3 as it's issues weren't quick fixes. So, a few weeks ago I got started. The only outward signs of an issue were a couple of bits of seam sealer in the engine bay lifting and a couple of rust stains breaking through. Scraping off the seam sealer and a slight prod with a screwdriver revealed this And removing the arch liner and giving the underseal a prod... The small round hole is a drain hole, in the middle of the square hole is where there should be a little plastic plug to hold the arch liner up. It's a common rust area, the blue compact is starting to go in the same area Having a feeling it would be pretty bad when I started digging I had already chopped some donor panels out of a compact someone local was breaking ( before I had decided to break mine ) So started drilling out the spot welds to see how the sections were pieced together. There's the upper rail ( which the wing attaches to the top of ) the turret and the closing panel which forms the main tub of the inner arch, and wraps around the inside of the turret and is welded again along the side. I didn't want to use the entire turret as it would need the engine to come out, plus there would be no way without constructing some sort of elaborate jig to be able to guarantee maintaining the original geometry. Thankfully my upper rail was perfect which meant it wouldn't have to be cut and would be a fixed reference point, I would then chop sections out of the turret a piece at a time only cutting out the minimum unneeded to to completely remove the rust. I made a rough cut to the under panel just to get it out of the way, it could be neatened and trimmed back later. And chose the first section of turret to remove. I picked the rear corner as it was already corroded through in the corner and not really doing anything to support the rest of the turret anyway. With that welded along its back and top edges and through the spot weld holes to the rail I could move to the next section which needed a far higher cut. I literally just skimmed the top of the rust ( upper right of this section ) there would be a piece behind which is why it's gone through the rust on the left side. New piece cut trimmed and clamped And stitched Onto the next section, daylight is never a good sign, This was where there was a telltale rust stain bursting through the paint. Thankfully I was able to start cutting back in towards the seam. The last piece, just needed a small section of the turret flange, but a larger chunk of the inner wing as there was some rust behind the brake pipe bracket, those two spots are where it was attached. And from inside Frankenstein With the turret solid and stable I could trim the inner tub where I wanted it, and trim the donor tub to suit Which was then plug welded through the drilled put spot weld holes from both underneath and on top, and given a tickle with a grinder Being impatient I gave it a squirt with an aerosol to make sure the general shape was ok and whether any bits would need more work. I then asked a mate who does repairs for a living what he used for underseal. He showed me a nice wurth gun, I did some reasearch and decided it was way out of my budget. Handily he had his old gun kicking around which he sold to me along with some sprayable sealer. I used the underside to practise, and tried to replicate the original ripple finish. And decided it was time to bite the bullet, mask it up and spray the visible seam inside the engine bay. Which has to be painted within 4 hours to ensure a proper bond with the sealer, so mixed up a little paint I had spare and gave it a coat ( this is purely to colour the seam and won't be the final finish ) A lot of it won't even be visible once the wiring, washer pipes, washer bottle etc are back in place And that is how it sits at the moment.