[Richard Coutts-Smith]

Another top tip is using a Copper backing plate. Piece of old electrical Buss Bar is perfect. Clamp behind and across the patch to be welded. As the weld will not stick to this Copper plate, it makes filling the join a lot easier, as well as sucking a lot of the heat out of the job.
Most Scrappies have drums of this stuff, and at pay per weight it is worth every penny.
As mentioned above, the extra thin cutting discs are a must, they are magnificent. The better fit a patch, the less welding, less heat, less clean up. Speaking of clean up, standard grinding discs tend to leave a pretty nasty finish. I use a softer compound polishing disc (they're white and I can't rember the brand, usually used for stainless steel work) after removing the worst of the weld.
Take your time, amazing what a mess that extra 1/2 second of welding can do.
Good Luck.
Rich.