I know of this mystery gummy substance of which you speak, and I use only Reverse Osmosis water in my iron.
Here's what I do, though perhaps you've tried this also:
Full Vinegar in the water compartment of iron. Iron set to hottest setting. Steam set to "on".
I remove my foam ironing pad, but confess I've never left it on when cleaning my iron
Get a clean bath towel that you don't mind ruining. I use a $1.50 Wally-World special.
Wet the towel and wring it out so that it still drips a bit, but not enough to make a mess.
Fold it in half, then in half again and place atop your ironing board. I'm sure you know what's next.
Iron the towel with good downward pressure hitting the manual steam button as you go (but don't break your ironing board...lesson learned).
Turn the towel over and repeat, refolding to the clean sides and repeating until clean. Be mindful that the towel is bustin' hot.
When done, I pour out any vinegar remaining in the iron and refill with RO water, and do a few shots of the manual steam button before cooling and stowing. I had to do this about once a month when hubs needed pressed pants and shirts everyday.
Wow, I'm so glad those days are behind me...
)
