I used iron plated tips on Weller irons, OEM parts. They last about 18 months of everyday use with regular multicore lead/tin eutectic, which is better than a brass or copper bit, but they don't survive forever. I'm talking industrial levels of usage though. I turn my own bits at home from brass rod, so if I have to junk one it's no big deal. The current one is over 2 years old though, they don't get used like one on an assembly line.
I've tried 3 different ways of registering double layer boards.
The first is to tape the first stencil to one side of the board and drill through a few holes or registration marks. Then you can line up the second side when you come to expose it. You have to clear out the drill swarf (dust) which gets trapped under the stencil when you drill through it.
The second is to tape both stencils to a strip of old board as a spacer, you can check the registration at that stage. Then you slip the blank board in between the stencils. This is a particularly good (low effort) way to go if you have a doublesided lightbox.
The third way is to cut the board accurately to size and match it up visually with the board edge on the stencil on the lightbox at exposure time. This is quite easy to do with phenolic-type board as you can score it both sides with a hobby knife and break it accurately over a table edge. You can polish the board down to size with a sanding block if you cut it marginally oversize, but I find I can usually compensate for any small inaccuracies by eye.
Edited by wakibaki - 4/19/12 at 5:29pm