A fair bit of this work will require a filler to smooth it over, and I'm reluctant to use much putty/bog. It chips and cracks easy and from what I've seen and read about never really lasts well. So, I have decided to use lead solder as a body filler to smooth things over, as it won't absorb water and is much harder to crack and chip.
Obviously being made out of lead it adds a fair bit of extra weight compared to standard plastic filler, but I'm building something to last, not a race car.
Also, just note, I'm no professional panel beater, just sharing what I'm learning as I go with the car so that others may benefit!
SAFETY WARNING: Lead, as we all (should) know, is toxic and can cause all sorts of nasty things to happen to you. Wear a dust mask or better yet, a respirator when performing this work and in a well ventilated area. Do your best to not breath the fumes in made by the tinning process, and don't do what I did and take a sniff at the jar of tinning butter. Your stupidity will be instantly rewarded by a headache, not good!!!

Anyway, on with the show!
Here's a basics of what you need to start leading: Tinning butter, wooden paddles to spread the lead out, lube to lubricate the paddles, something to spread the tinning butter on your project, bi-carb soda to neutralize the acid in the tinning butter, and lead body solder (which is a 70:30 mix of lead to tin for automotive work if I remember correctly). This is the basic Eastwood ( http://www.eastwood.com/ ) kit I bought off Ebay.
What I have not shown that you also need is: a source of heat to melt the lead and tin the surface to be soldered (I use a primus lpg torch and 9kg lpg cylinder. You could also use a bernzomatic or oxy/acetylene torch too), a body file (found at most body repair shops, ask for the finer toothed one, I think mines a 13tpi file), water and rags. I also used some 100grit sandpaper to finish off the peice at the end, but there are a few safety warnings for that too, so stay tuned!!

The guard to be modified:

The hole to be filled:

Here I have a piece of 1.5mm zinc anneal sheet steel that I will cut out to plug the hole.

Tacked in. Take your time welding sheet steel. Its easy to warp/blow holes in if you have too much amps/wrong technique or try welding too much at a time. And if you have access to a gas-fed mig, make use of it! It really is the easiest way to weld anything for the average home user. pretty much kept putting small tacks onto the job and worked around it in a star pattern until it was all welded, and took breaks in between to cool it down using a damp rag. Oh and if using zinc-plated steel like I did, find a way of removing the zinc coating on the side to be welded. Zincs a ***** to weld on without good amounts of amps.

Welds all tidied up and ready for lead. And yes, I know I didn't get the plug tacked in very straight....

Apply your tinning butter. I didn't do so well tinning this one, and I think its because I may have used a little too much paste and when I heated it the lead just melted right out of it and rolled off the surface and didn't etch into the steel. Next time I will try less and on an even surface if I can to see if it helps...

Now using your heating-torch, slowly and evenly heat the area to be tinned until the butter kind of "boils" and goes a golden brown (sounds like a cooking recipe lol). Then let it cool a little and wipe the area with a damp cloth to remove the butter and check if your efforts have been effective. I had to do this twice to get the coverage I wanted. Then mix up a paste of bi-carb soda and water and spread it on the area to neutralize the acid in the tinning butter. Otherwise I'm led to believe this won't be so great for your painting efforts later! Sorry I have no pictures of this, bit of a rush and hard to do whilst your holding a hot flame!
I also then went and gave the whole panel a good rinse with water, just to make sure I had removed all traces of the acid.
The results of the tinning:

By the way, in case you were wondering, if you didn't tin the area to be soldered the lead would not stick to the steel. The tinning butter actually chemically bonds to the steel allowing the lead your about to apply to it to stick. Same goes for any brazing or soldering (electrical and silver solder usually comes with its own flux built into the rod).
Now, time to apply your solder! This is a little tricky if you haven't worked with melting metals before, but you need to keep the lead from going runny. What your looking for is a "peanut butter" consistency to the heated lead, so that you can work it and spread it across the surface without having it liquefy and running off the job. I found I had best results by heating the surface up a little, then putting a fair bit of heat into my lead stick (the ones from Eastwood are thick and take a bit to heat up!) then "mashing" it onto the job. I was then moving the flame off and onto the job a bit and in between using the wooden paddles coated in the paddle lube to spread the solder over the job. Try to get it as even and smooth as you can, means less filing later! Here's my results:

Now, just start filing away until you have the shape of the panel you are repairing! The body file I am using is adjustable so you can file to a concave or convex shape. I'm sure you could get away with a regular coarse file but this without a doubt makes the job heaps easier and neater! To get the results below, I used some 100 grit sandpaper. Please, if your going to do this, wear a dusk mask at the very least to avoid poisoning yourself by breathing in the air-born lead, and WET THE SANDPAPER AND JOB!!!!! By KEEPING the job wet, in theory any of the lead dust made from sanding SHOULD be trapped in the water, and you can end up with a very smooth finish that requires little extra prepping for paint. Again, use lots of caution and sense here. DON'T use power tools on this stuff. Sorry for all the warnings, but I have visions of someone attacking a leaded job with an angle grinder whilst their kids are in the yard if I don't include them

The results:




Just needs a little bit of dressing with some putty/high-fill primer, as there is a few small grooves where the tinning wasn't effective, but overall I'm very happy with the result.
Hope that somebody benefits from this, and sorry for the long story!
Jordan