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I do a lot of forearm work as well and have luckily never had anything.
But when i get a dermatitis anywhere else peripherally, i usually use a high end pure lavender oil on it and it
tends to go away pretty fast, being the antibacterial, anti fungal that it is. I do not dilute.
otherwise, my next stop would be a derms office, since you don't know what it is. It could spread to other areas as well as to others.
Best of health to you and hope it clears soon,