I'm using XCR coating resin since last year. And never had difficulties to sand it dry. If it was hardened for at least two days, dry sanding always went smooth. But I've experienced the opposite this Summer. When it was impossible to dry sand resin that was hardened for 4-5 days. It clogs sandpaper or Abranet immediately. Also noticed weaker adhesion properties of that resin, no matter that previous coat was sanded with 220 paper. I have suspicion that it has a lot to do with air humidity. But maybe there are some additives available that would help to avoid such problems ?

Seems I found an issue . Main difference between that coatings in spring was that it was on laminate of the same (XCR) resin. These past few times it was on laminate of EL2 resin. I thought that your EL2 resin will be compatible with XCR. It seems that not completely. I've experienced similar issues last year when XCR stayed not cured on MGS285 resin. It stayed very sticky for a very long time ( 3 weeks ). Then I tried to scrub and wash it.  I saw that XCR is very sensitive to what lays underneath. So I decided to switch to EL2. Finally see that it does not work also. I loved all XCR properties except of that sensitivity. It's sad but I'm starting to look for another system.