…. rim wax? I believe that Tioga make one but can’t source it in the UK.
Apparently, it makes mounting tyres a bit easier. I tend to have trouble with getting mine to seat correctly on the odd occasion, despite best effort in jiggling it about on the rim every 5psi and/or blowing ot to 110% recommended max. pressure and bouncing it around. 😕
Problem seems be v-e-r-y slightly easier with steel beaded ones.