Grabbed the first piece of metal that came to hand, some 24ga galvanized, and let it rip. Bad choice. Should not TIG weld galvanized metal. Besides the zinc oxide being rather poisonous to breath, it makes a mess out of just about everything else. Created this bizarre mothy web of something on the back of the weld, and totally corroded my tungsten. It got so bad that the high frequency kicked into overdrive and started sparking from the collet. That was after it had burnt a nice hole right through the sheet. Cleaned up the tip, turned the power down a bunch, and finished a line, just to remind myself that it was possible, but it wasn't pretty.
Grabbed some 22ga paintlock next and started practicing some lines. Blew big holes on the ends at the beginning, but got more careful as I went along and could end a line at the edge without major melt-through by the time I finished the sixth one. I also tried out the pulser mode on the HTP, but mostly that helped me make more of a mess. Forget the pretty pictures in the books (see above), if it wasn't too slow (and completely solidifying between each blip), it was apparently pointless (making extra crud on the surface of the weld). The bead didn't look any smaller, and the penetration didn't look any better.
At about 12Hz (30%) I was actually doing ok, it was giving me somewhat well defined melting spots, but one thing you don't realize when reading about pulsing in the book, is that the only light source for your work is the TIG arc. If its pulsing, you're essentially working to a strobe light. And trying to do anything with a 12Hz flash in your face going: blat blat blat, is mighty distracting. Supposedly the sound gets really annoying as well when working with thicker work, but since my upper end is 200amps, I'm not likely to be doing any serious ironwork.
There's some instruction on using pulsing for TIG here