I do a lot of etching with PNP, and have given workshop in the
process. One thing is certain. Each iron heats differently so it was
not helpful to specify the precise temperature. I have the students
start with a medium temp then raise it so that it is hotter, but not
hot enough to melt the PNP. This will take practice until you find
the perfect temperature for your iron.
Regarding bubbles. First of all, I constantly move my iron around,
pressing down firmly. My PNP has been cut and placed so that I have
one corner free, so that I can lift it and peek to see how well it is
taking. When it looks like it has pretty much adhered, but I have
some bubbles to deal with, I begin the burnishing process using my
metal burnisher. I burnish thoroughly, pressing out any bubbles then
I heat again with my iron. If necessary I burnish again. Burnishing
is an important part of the process, so don’t neglect it.
Some people like to burnish with a piece of wood, covered with paper
towel which has been fasten to the wood with masking tape. Me? I just
use my metal burnisher.
I don’t use a griddle. I just have a piece of wood, covered with a
heavy piece of cardboard, and use as a base.