Don’t get me started on this, I’ve suffered with it for years!
It’s also known as Hikers Rash. I’ve been to the doctors with it a couple of times and you’re right, there’s no cure and nothing seems to work that will prevent it completely. It also apparently affects more women of shall we say mature age
I have worked out over the years that for me, although heat is a factor, sun isn’t, but humidity definitely is and worse than just heat. I have had it with long trousers and shorts, and have tried different socks. I think like you mention, the terrain is also a factor. We have walked in Snowdonia and the Lakes this year and although fairly hot and I might have expected to get it, I only really got a few spots around the sock line, but when I have had it really bad, it’s been very red and painful and almost up to my knee. The ground was mostly rocky, and I seem to get it most of all when humid and grassy. It’s ok if only walking one or two days but we have done quite a few long distance walks and if it builds up, it can really be quite painful. The worst time was on the Cotswold way, whereas the Pennine way it barely occurred at all.
So what helps? Nothing works completely but several things seem to ease it. I rub nappy rash cream into my legs before starting the walk. If you start to get it during the day, dipping your legs in a cool stream is a good idea. After the walk aloe vera gel seems to sooth it most. I also got some antihistamine tablets (fexofenadine) from the GP which if I start to get the rash, I will take one before heading out. That’s all I can think of for now, if I can remember anything else I do I’ll come back to you. Good luck!