LOL it's the first time i read about this Chris, thanks! amazing, the human body
i've heard of "elbow grease" but that's something else...
On my topic: well as i found out ordering and delivery of spotting pigment(s) would take a lot of time and as i am a not very patient guy in my hobby, i decided to go ahead and test just about any mark-leaving utensil in my household on some scrap photos that have the same tone and surface.
so i found out that a black colored soft tip pen marker (felt..tou know the stuff...) does get the scratch blackened sufficiently (it was NOT a deep scratch but it looked white, i guess due to scuffed-up emulsion and visible paper base). Only, it also leaves a deep purplish surface sheen behind.
if i paint the scratch and then immediately wash the surface thoroughly, with wet fingers rubbing it, the marker gets removed from the print surface and it stays behind in the scratch only.
So i did just that, and the result is excellent
Of course the PE surface is still scratched so if you look at the print in a grazing angle or with a grazing light the scratch on the smooth surface is still visible. But for normal viewing it's not white anymore, it blends into the background tone.
So, yey, i found a way.
Thanks for the suggestions ; i will get a spotting set anyway as i inevitably have sometimes a dust spec leaving a white mark, these do not bother me too much but sometimes it gets in the wrong place.