I guess the most unusual request from me, was that I do a couple of painitngs on slate, for a man that was going to re do his front doorway with slate. He was in the Lions Club and wanted lions on one piece and some other "big cat" on another piece.
I had no idea what to use, but after a bit of checking I found that I could use acrylics if I sealed it properly with an outdoor varnish. He was told he may have to put on a new coat occasionally if it started looking worn. The pieces would be protected by a roof so they would get little exposure to the elements.
He was thrilled, and as far as I know he has had no problems, that was about 15 years ago.