Thanks Werd for the link!
Had a look at the video, it seems it's for rubber coated metal trim rather than solid rubber.
There doesn't seem to anything on youtube but I've found this on a forum post about 97 prelude, random but seem to have the exact issue I've got on my XJ......
https://www.autogeekonline.net/forum...ndow-trim.html
From reading through the posts I can make out its some kind of fugus or rot attacking and 'scabbing' the rubber and you need too:
1. Kill the fugus/rot with a cleaner thats not going to stain or fade the rubber. So not bleach.
2. Rub down the rubber trim in small sections at a time with a dish washing pad.
3. Then final treat the rubber with a plastic/rubber trim treatment.
Although expect this could be a temporary fix and not really be that good. Probably get a new windscreen rubber trim would be the easiest.
I'll give a go hopefully this weekend.