Tap water is bad, unless the mfg clearly says it is ok. Mineral deposits
can build up inside the iron, blocking the steam vents, possibly even
the water feed to the steam plate if it's allowed to sit.
To clean your sole plate, go to any fabric store, look for the notions
wall, where the pins, needles, snaps, etc are. Look for a product in a
tube called Iron-Off, made by Dritz. It is a creamy paste, won't hurt
your soleplate and will remove any sort of gunk. I've been sewing for
years, and always end up with residue from fusible products, starches,
etc. This stuff has NEVER failed me; been a staple of my sewing supplies
for 20 years. It's about US $5 tube. One tube lasts me well over a year.
To use it, first get your iron at hot as it will go, no steam. While
it's heating, squeeze 2-3 inches onto a thickly folded bath towel.
Once the iron is full hot, TAKE EVERYTHING OUTSIDE or the smoke will set
off your smoke detector :) Your iron will stay hot long enough to get
the job done. Then rub the iron around and around in the paste, and the
gunk and the paste go up in smoke. If the iron is really bad you may
need to reheat the iron and do it again. It helps to put the towel on
the ground, so you can apply a little muscle.
Tips...I've found a terry cloth bath towel works the best; the rough
surface works better than a smooth cloth, for instance an old t-shirt. I
haven't had it ever stain a towel, but I've always used old rag towels
anyway...so don't use you good bath towels.
I've heard it works for soldering irons, too.
(Amazon.com product link shortened)