patinas

I just paint the PE.  I think it would be very difficult to control getting the right patinas by chemical agents.  What metal are you trying to make the PE look like?  In case of small recreational boats that use a lot of brass, you could just make brass PE go unfinished.  Chemically coloring stainless PE is a lot harder and unpredictable.  There are any number of stainless steel alloys.

Steve You might try Blacken-it. works great on brass PE.You just dip the PE in for a few minutes and it turns  it black.dosen't chip.I've found it on Amazon also Micro Mart carries it. I've had great results with it..

Rod

Blacken-It is good stuff. Bluejacket also makes three toners: Brass Black, Pewter Black (for Britannia metal) and Brass Brown. I've also seen some stuff called "Patina-It, but I haven't tried it.