Back when I was weaving, I did a series using sisal and rusted or copper patinaed papers as well as found rusted elements. The unryu paper was rusted/patinaed using instant rust/copper solutions purchased from a craft store.
I'd lay a piece of freezer paper down first to protect my work surface. The solutions usually bled through to the freezer paper. Two pieces of it were so lovely that I just couldn't throw them away. Or figure out what to do with them.
Last week I kept looking at the largest piece of rusted freezer paper, trying desperately to remember if a matte medium had been applied. You see, the temptation to discover what would happen if I attempted to rust cotton with the freezer paper was too much to resist. Cotton was dampened and sprayed with a little vinegar then folded with the freezer paper and all was placed in a big baggie to be put outside in the sun...in hopes of speeding up the process.
Still can't remember if a matte medium was applied, but am betting it was. The cotton is rusting in places, but not overall. When the rain began today, it seemed the ideal time to experiment again. The cotton and rusted freezer paper have been removed from the bag and are laying on the table, soaking up the rain and hopefully propogating rust.