GO FOR IT
i have done seven different church(es)? windows and have become excepted as
"as a specialist".
First time was a small chapel attached to a large country house. Used wfp outside and it came up like new.Then spent three long dirty hours mopping up the mess inside.
Yes they do leak, so i now always say inside and out only.
We use a small trough-like set up with with a doubled over rubber edge attached, place against bottom of window inside and this catches around 90% of the runs.
Pre-soak from the top down with your pure water as if you were rinsing off, leave couple of minutes but don't let it dry then wfp with slow water flow and gentle brushing in a horizontal fashion.
The dirt just runs off and excuse the pun but God do they look good
Inspect first for broken or cracked pains agree the damage, be slow and careful and charge what you like, these institutions are not short of cash when it comes to there buildings.
Careful, slowly and plan
Cheers
Gordon