Your armpit hairs grow in various directions. So shaving in a particular direction can cause skin irritation. When shaving, move the razor both downward and sideward too. Also try to shave in other direction too, if your skin is not too sensitive.
Each time you shave, your razor picks up dirt, shaving cream and bacteria, making it difficult to use the blade. So it is highly recommended to rinse the blade each time to get rid of these bacteria and dirt too.
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