If you use an anti perspirant then make sure it is clinically recommended for the face and scalp.
Generaly those that are designed for the face and scalp(maxim,odaban etc..) should be applied at night and washed off in the morning.
I wouldn't recommend any product that is not designed or approved for the face and scalp as the chemicals used might do some harm over time on far more sensitive skin there than the underarms.
Paul thanks for the link but scalp protectors do not stop sweating just form a barrier for a short while and i have found to be totally useless in a hot and humid enviroment.