Thought I'd inject a comment here. The Nickel paint is an excellent solution.
In a thin trace, it is quite flexible (+/- 40 degrees plus flexure). While I
have used fingernail goop on desperate occasion, there are good and
inexpensive insulating dopes available at the same source that you will have
to go to get the Nickel paint. BTW, a Silver paint is also available for more
critical environments. Of course, the surface (after removal of the thin
plastic overlayer) must be cleaned. Good, residue-less cleaners are also
available at the electronics parts store. I have about twelve foils in
various stages of modification heavily gooped in this fashion. No failures
over the last 10 years.

That said, I passed this advice along several years ago. I was surprised to
receive a desultory missive from a former list member as to how the Nickel
paint wouldn't adhere to the foil. Seems he was able to make small extensions
of the copper trace which would float above the foil! Upon further
investigation, I discovered he had cleaned the foil with WD40! Good grief, a
release agent.

Frank Grunthaner