just a random thought but artificial sweeteners can cause a sun sensitivity- haven't had much burning since I quit using sugar and sweeteners.

I rode 50 the other day without the heat rash- I gooped up with everything balm (all natural bee wax based with eucalyptus,tea tree oil, aconite, rosemary, lavender aloe and calendula) as a base layer,and then layered sunscreen over that. It worked so will continue and see if it continues to help. I am a very salty sweater so that may be part of the problem as well.

Anyway, thought the thing about sweetener might help as well.