Twit,you are correct about it being on mostly the older larger stems that hang with all the new growth on the bottom.I do mist,i haven't had any problems with my Sulcata!! But my big old man which i have adopted calling him since not sure exactly who he is.I seen a picture of one that looked a lot like mine and one of the common names was "old mans beard"has a good many of these crude places and the first one i noticed is the thickest. I also think you may be on to something with the build up because every place is only on one side of the stem,side facing out and the stem's all seem unaffected by it.The stem with the worst pace is growing great and has already gotten so much new growth this year.I also put all of my HB's in this shower to give a good watering and also great way to dust the leaves.I put up an extra shower curtain bar & hang a few from it at a time.The Hoya's & Rhip's really seem to like it.I will take your advice on misting with distilled water,i let my water sit out 3 days before using but i guess that isn't enough.
I was worried about your 3rd. theory!!! But i believe you have something with the build up.I have another picture of the oldest spot.It reminds me of an old clay pot that has so much build up on it that you can scrape it off with a knife.Starts out light & smooth and then in time this is what you end up with.
My R. Sulcata turned a pretty red mostly on the newer growth during the summer.I keep it in bright shade,it seemed to sunburn easily when in to much sun.It did get more sun right as the sun was going down.
