stains

Hey guys, I hope everyone is having a great day. It's my first post here (definitely not the last), and I'm kindly asking of your thoughts about an issue we had.  We recently cleaned a client's UPVC windows internally as part of a deep clean job. The window sills and frames were very dirty...

Has anyone ever come across this before, the customer said the builder caused it either from acid or it's from a metal strip that runs above the patio doors that has caused dripping stains. It looks like to me staining from hard water but they assure me it's not. It's on the glass and frame...