@fernandez My German is not very good, and I read this as "Are you kidding? This will never go away" or "This will never clean off".

To confirm, I ran this through two online translators (ddg and google translate) and god wildly different results, none of which made sense.

I'd love it if someone could help me translate this.

@loke @fernandez your translation is correct! Except I'd probably say "Are you nuts? This will never clean off!" instead of "Are you kidding?..."
@loke Translation is ok. I’d say it’s „Are you nuts“ instead of „Are you kidding“, because „Spinnst du“ is a lot more condemning than „kidding“, it makes very clear that you’ve done something intolerable.
@loke @fernandez yes, that's about what it means.