rust is a killer

a while ago one of my plants developed rust colored spots which stared @ bottom of plant and slowly worked its way to the top. I flushed the 1 gl pot w/about 2-3 gl of tap h20; got rid of the larger affected leaves, and let the plant sit outside. New leaves started sprouting, and a week into it, they're developing the same exact rust problems! WTF??? The rust spots start small then take over the whole leaf and render the leaf very dry!

Any suggestions?

Well, in case if anyone re-reads this, here is an update:
I took some drastic measures. I kept this plant apart from my other one (only have 2 matures). Then, I decided to repot. I threw away the old pot, took the plant out and rinsed out its rootball (with the help of a hose and low-pressure running water) so that it would be left with very litte of the original soil left. Then, I replanted in miracle gro potting soil mixed with sand. The plant's growing and sending out new leaves w/out the brown or rust spots.

Musta been nute burn or a deficiency.