On Feb 26, 1:51 pm, Ross wrote:

As so often said here, pardon me, what is the reference for "not
permitted"? Is is a Piper document or what? I would like to make it
available to the owner.

I believe this is written in the Piper PA28 Service manual itself. I
have the Essco Aircraft reprints of these manuals at home and can
probably look up the exact section, page and paragraph later this
evening. I distinctly remember reading this info in the manual, as I
was looking to buy a Cherokee that has a skin patch on one of the flap
skins and wanted to be sure it was an approved repair.