ah, I hear you. 'prickly heat' as the Ground Effect man put it when I talked to him at the Cycleshow.
the sad thing is for those of us with sensitive skin, the wonders of merino need to be midlayer - and you have to stay with polyester base layers. DAHIK. (tho strangely the Swrve WWR shorts have a mild irritation factor as well for me and another who has tried them. we loves them tho so we puts up with it. oh yes.)
some manufacturers are putting a thin layer of polyester inside the merino, but then you lose a LOT of the wicking capability :( and pretty much shouldn't bother.
I'm trying a variety of merino stuff atm, they are subtly different - I might find one I can cope with by next winter. Carradice had a new merino jersey on their stand and that was much better than others I've tried on - but that's not a base layer.
As I mentioned above go with polypropylene not polyester, it's much better at wicking.
As I mentioned above go with polypropylene not polyester, it's much better at wicking.