Filler on UPVC Window

[bazwod]

Hi all, have tried to find the answer, but can't find a solution!!

We have UPVC windows and when they've been painted or put new window sills in the last people that lived here made a right mess up!! There is a combination of both paint and and filler (not silicone but more of a pastey/powder based filler) all over the window frames. I've tried white spirit to get it off with no success. Don't want to scrape it coz it'll scratch the upvc.

Any advice greatly appreciated!!