One of the downsides of spending time at the beach is that we occasionally get a heat rash on our chest. That was until we learned how to prevent one. After a particularly bad case of heat rash in Mexico a few years back, we asked Dr. Doris Day, our dermatologist, for advice on a heat rash cure. She informed us that heat rash is actually an allergic reaction and suggested we take an OTC antihistamine, like Zyrtec, in the morning pre-beach or other sun exposure.
Well, we did this on both our recent trips to Grand Cayman and Barbados and it worked like a charm! No ugly red bumps, no prickly heat.
Obviously, we're not doctors, so we can't promise it will work for you. But it's worth asking your own derm what she thinks. Of course, just because you get a rash, doesn't mean you can go without sunblock!