It probably will not work as well because the hair present on top of the skin would absorb more light energy. The light energy needs to concentrate below the skin as much as possible because that is where the hair growth takes place. Also, you may damage your skin with this energy if you do not shave these dark, full length hairs. You do have dark, coarse hairs on your arms, right?? The directions are there for a reason and I would follow the directions as laid out by the manufacturer and the doctor selling this device.