Hiya, my dd2 also 8 has a handwashing obsession also.
Tbh hers started at around 4 years old and slowly increased in intensity until she was about 6 and I was having to smoother her hands in cream as they were so sore.
Dd's obbession is also about germs, and she is particularly obbsessive about hand washing when using the toilet. I know that sounds perfectly sane, as you need to wash hands after using the toilet, but it got the extent with dd, that even if she went into the bathroom, and didn't use the toilet, she'd still have to wash her hands. She also developed a habit of sniffing her hands.
In the end I went and saw my gp who reffered dd to a psych. The psych was very matter of fact with dd, and basically told her that she was not allowed to wash her hands unless she had actually been to the toilet or had been doing some activity (like painting) which required them to be cleaned afterwards.
I was also instructed to stop dd from washing her hands if I found her breaking the rules.
It was very hard. Dd accepted what the psych had said but did try and break the rules and I had to practically sit on her a couple of times to stop her.
2 years on, she is alot better but the obsession is still there and flares up every now and again, normally when dd is generally stressy about something. She still washed her hands like a surgeon and will scream at ds at the top of her voice if she catches him leaving the bathroom without washing his hands.
She doesn't see the psych anymore, she was discharged after 2 session, and tbh I wasn't overly impressed with the treatment we recived and so was happy not to go back.
It does worry me that dd might get obbsessional about other things when she is older, but the psych told me that I simply have to tell her it isn't allowed and physically stop her.
I would definatly go and see your gp about it, even if just to talk it over.