What must someone afflicted with hyperhidrosis do, especially if they do not feel the water of ablution on their hands and doubt its reaching the skin or under the nails due to their illness or residues of soap and shampoo?

What prevents water from reaching the skin must be removed during ablution (Wudu') and ritual bathing (Ghusl), based on Allah's saying: ﴿So wash your faces and your arms to the elbows, and wipe over your heads and wash your feet to the ankles﴾.

The rule is that anything with a physical mass that prevents water from reaching [the skin], like nail polish, must be removed. However, anything without a physical mass, such as the color of henna or the residue of oil, does not need to be removed, and purification is valid with it. The smoothness resulting from shampoo and soap is not considered a barrier. One should beware of obsessive doubts (waswasah).