Dd2 had/has a very similar problem, but with her it is an obsession with washing her hands not drying.
Hers started when she was about 3/4, and got to the point where she would have to wash her hands if she had just touched the toilet but not actually used it, if she'd touched anyone else who had recently been to the toilet or if she for any reason thought there were germs on her hands.
She tried to be secretive about it, but i'd constantly catch her washing her hands for on good reason, and if I stopped her then all hell broke lose. We once stopped at a loo on a motorway and there was no soap for dd to wash her hands. She screamed non stop for an hour in the car.
I was at a complete loss as to what I should do. I mean logically, dd was right in that she'd wash her hands after using the toilet etc, but she'd talen it way too far. I was unsure if I should just let her do it, ignore it or stop her.
She was reffered to a psych who tbh was pretty rubbish but insisted that I must stop dd from washing her hands unless she had actually been to the toilet and done something on it. I was told I had to physically stop her if needs be, and I did have to, infact it wasn't unheard of for me to sit on her to stop her washing them.
After a while dd did start to get better, but it has never completely gone away. She still washes her hands like a surgeon getting ready to go into theatre and when she is stressed it does briefly flare up again and I have to remind her of the rules, and you can see she hates me for that.
I would suggest that you speak to your gp and ask for a referal to a psych, even if just for advice on how to manage it.
It is hard to deal with but I would say for you to try and be quite matter of fact with it all, and try not to get upset or anxious yourself as she will pick up on that and it may make it worse.
Good luck