Cheers for the comments guys! The interior was off this forum a while back - really happy with it.
I eventually found the pictures of my attempt at arch repair (note to self, stop being lazy and scattering stuff around in "NewFolder").
I'm not too sure whether to be happy/proud of what I've done or a little bit embarrassed - I guess I'll leave it up to the replies to the pictures before I make that decision ;)
I didn't take that many pictures at the time as didn't think I'd be documenting it so I'll fill in the blanks.
First off, the extent of the rust/rot on the nearside rear after I poked away at it to get the really crumbly stuff off and after a bit of a sand back:
And another one from the inside (hello word!
The next step needs a brief explanation. I am a software engineer (nerd) by trade and at the time a poorly paid nerd, my ability with a welder is none existent and I needed to sort this one out pretty quickly and cheaply so that it was at least water tight. As such, my tools of choice were limited to what Halfords could provide - doomed from the start you might say!
The eagle eyed of you will have spotted the corner of a pink square at the bottom of the inside view of the rust problem. This gives a hint as to my approach and the pink corner is my failed attempt!
I decided to go the fibreglass route. Before cutting the rust out, and not knowing how much of the arch I'd need to cut out, I wanted to make a "mould" of the inside out of fibreglass. I'd then bond this back onto the inside once I'd cut out the rust and this would hopefully make building back up with body filler retaining the shape that bit easier.
I guess now would be a good time to point out this was my first attempt working with fibreglass as well as body filling.
The problem was a suitable releasing agent to ensure I could take the mould off so the rust could be cut out. Halfords didn't seem to do that sort of thing. After a bit of research it seemed that main component of normal releasing wax was carnauba wax - I have some of that!
The picture showing the pink square is testament to no, carnauba wax meant for car exterior waxing is NOT a suitable releasing agent! That square is now permanently bonded to the car.
So, for something a bit more robust. In the end I went with what I had in the house - get ready to cringe...
I used ordinary table candles melted down over hot water and applied thinly to the inside of the arch. It was horribly messy and wrote off the kitchen for a few days but did the trick nicely.
I then built up numerous layers of fibreglass and fibretissue until a nice mould was created. With a bit of effort and hope it popped off cleanly when set. A slightly tricky job manoeuvring it out of the arch past the inner skeleton but with a bit of wiggling it was out.
This is the result:
Next step was removing the infected metal back to something decent. For this I used one of those little dremel replica tools. With it being a replica the speed control broke within 5 minutes so was left using it at maximum speed!
Worked very well though and allowed for a pretty precise cut using the dremel (real this time) reinforced cutting discs.
One from the outside:
One from the side:
In the end I didn't actually have to take too much of the arch off only really the outer edge.
This rendered my mould attempts pointless to some extent but I still bonded it in anyway as I'm sure it must help in some small way, maybe.
The next bit was just using some wire mesh and a combination of Isopon P38 and P40 built it back up again.
Next was a proper rub down of the area I wanted to repaint. Oh, minor but I also ground out and filled two smaller rust spots at the rear of the arch.
Looking back at the pictures, especially the one below I can't quite understand why I decided to take so much back to bare metal. I think there were a few spots that had been touched up badly I wanted to include, but it may have just been a bit of a mad day - one of the two.
I can't remember what grit level I went down to in the end, 400 or 800 I think. To check I had the arch blended right it seemed quite useful to wet the area then look down the side of it. The shiny surface showed up any pinholes and also any bad blending.
Then a few layers of primer, Halfords zinc stuff I think it was - the can says its supposed to help stop rust returning, we'll see. Obviously goes without saying that preparation was key to a good coat. The Halfords cans need some serious shaking, I mean loads.
If you don't shake it enough you get a really inconsistent flow. Good to find these things out on the primer layer rather than colour/top coats
Shake it about twice as long as it says after leaving it in a bath of hot water for 10 - 15 mins. Shake it until your arms can't take it any more, then its done.
I also used one of those tack cloths to remove any dust I could and did the spraying as far into my garage as I could after soaking the floor a bit with water to keep any dust at bay.
It may not be completely necessary for the primer layer since its sanded down a bit after drying but I wanted to work out the process for the colour coat.
Here is the arch including primer:
Sanded down again - I actually went too far and had to reapply primer so be really careful. The point that the arch lip begins is a nice point for any sanding block you are using to pivot on and sand much quicker. I resorted to really light hand sanding in the end to avoid this.
And then the colour coat. This was again Halfords own KH3 spray can. Again, same rules for shaking apply.
I still need to do the clear coat. In fact, I need to sand down the back of the arch and tackle the rust that has reappeared. The part I cut out and replaces with fibreglass is looking good and no signs of any rust returning but it looks like the spots I tackled at the rear have returned
So, a still as yet unfinished repair, but I think I am so far pleased with how it has turned out. I'll need to sort out the bits that have returned which is a pain but once this is done and the clear coat is on I don't think it will be half bad. The colour difference is there, but I think with a bit of blending that should become a little less obvious.
So, should my hands be severed to avoid further incidents such as this inflicted on my Skyline or does it look reasonable?