“Mylar?”
Yes. I use the ‘one side mat finish’ - it has good tooth...and can be ‘fixed’ with workable fixative as it progresses.
Holds the colors true - and is archival
(I like to experiment - I like Bristol board for pencil -
and I do silver point - have to prepare surface with several layers of gesso (easier than making rabbit glue and grinding down bones for powder like the Old Masters! ;)...)
I work with ‘mylar’ every day in my job.
It’s a thin heat resistant plastic used in electronics as an insulator. It can be in sheets or on a roll like masking tape. It is also bought in pre-cut forms like washers and squares for mounting transistors that get hot, since it has excellent thermal properties.
It is also used in manufacturing electronic components, capacitors for instance, since it is an insulator and is very stable over time and heat exposure.
Is the mylar you use clear when new?
If so, then after you paint the portrait and it has dried, try shining a light from BEHIND the portrait some time. You may be pleasantly surprised...................