[Geezer (OP)]

The seals on some of my "sealed" double glazed windows have failed allowing moisture to enter the space between the panes. Now there are ugly whitish streaks running down the inside surfaces of the glass.

What causes the streaking, and what fluids can I use to remove it?

(I can drill small holes in the top and bottom of the outer pane then install valves.)

[LeeE]

Yeah, this is a very common problem.  I've always assumed (but don't know) that the streaks that are left are akin the the 'fur' you get in your kettle', left behind as the water evaporates.

[chris]

I'd be a bit worried about drilling a hole in the glazing though - that seems a bit draconian... Why not just replace the windows?