If you are with baby constantly and he starts to drool excessively, you can keep a burp cloth at hand or attached to baby's top so that you can wipe off the drool as soon as it comes. Wipe gently so as not to aggravate the area.
To treat it, on top of what Jacq Ng says, gently wash the rashy areas with warm water, then pat dry. Again, don’t rub because it can irritate an already sensitive skin. After dried, I read that applying petroleum jelly (Vaseline for Baby) will help. It will act as a barrier between your baby’s skin and the drool :)
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