Side windows

[johny transit]

Just picked up a pair of slide open side windows (for each sld), but was surprised to find they were full of mastic in the seals. From my previous experience I thought from factory these were a dry fit - and these came from factory fitted Mk4 minibus/tourneo. Wonder if someone had problems and gunned some mastic in later on. Otherwise seals look good as far as I can tell.
Also - I think I am gonna have to really clean them up good before attempting to fit - can't see the draw string method working with all that gunk, but should I mastic them again after? Or worst case get new seals? I saw the bus they came off, no signs of rust around windows to indicate that anyone had had trouble with leaks. What does anyone think?
Thanks