i think trad and wfp its hard to get absolutely perfect results, when using trad i often found that if you left a mark in the middle of the window with a squeegee and went to wipe off with the cleanest dryest scrim you would be hard pushed not to leave some kind of smeary mark and would often be easier to just soap up and do again, i think imo scrims do leave marks and because when you dont leave any marks you only use them for detailing the edges and theyre just not so noticable there.
ive had the odd custie complain about wfp now which is hilarious for some of them coz i know for them its a much better job coz i couldnt reach all parts of top lights etc when trad coz just too high or knew that leaded or georgian chances are there would be smears somewhere if you really looked.