i, I can understand your concern for blistering red rash on forearms and knees. There can be multiple possibilities of the red rashes and it requires a close physical examination to diagnose the exact entity of the problem. In my view correlating with the facts you have provided it appears to be allergic dermatitis of the area due to multiple causes. I suggest you to meet a dermatologist to for proper therapy. In between you should avoid contact of dust to the area properly. If I were your dermatologist then presumptively would have preferred to apply fluticasone cream. You can take an antihistamine such as loratadine for twice a day if itching is present. Do not try to pinch, pop or squeeze the area as it may cause secondary infection of the area. All the best