No finish necessary . Chemical layers interfere with the refraction (colours) . I have some sealed plexiglass frames to completely isolate the copper from oxygen , but haven't found them necessary after 6 years of careless storage , still vibrant as ever !.

Hi Nelson ,
name's Colin , and thank you , It's nice to be among artists of such high caliber here. Re ; heat tinting , the flatness of the sheet is essential for lasting and varied ' rainbow colours ' , just as the flatness of puddle water with oil on it will produce beautiful light refractions .
As I was saying in the above comment , exposure to oxygen is the key factor , "greening" copper is a very slow process even in outdoor conditions as you probably know . Anneal and "fire paint " your sheet , keep it indoors and it will preserve as long as any comparable medium under similar circumstances .
As far as sharing the techniques I don't mind because A) There is so little real interest in what I do that it's not a danger ;) B) I challenge people to take it further than me! Show me something new I can add to my repertoire . C) that's what I figured this site was about , exchange .
I appreciate your sound questions and hope I answered them clearly , thank you ,
Colin

wow...and thanks for sharing this with us. I've never seen anything like it before. Your "painting" is amazing!!!
beautiful colors...I don't work with copper but would like to experiment with this someday!!!!SteelyJan

I hope you do try this out , all you need is

1. copper sheet 12oz
2. propane torch
3. conventional oven .