Thats why it pays to spray dinitrol cavity wax into the sills via the drain holes and the trim clip holes (if you can get them off without breaking) - however if tin worm has taken hold it delays the inevitable - and that is a welding fix.
But if your sills are OK get them filled with dinitrol. A good indicator of sill health are the jacking pads - if you remove them and they are clean then the sill is looking good - if the pad is covered in rusty gunk you know the tinworm has struck. Usually strikes at the rear of the sill between the sill end and the jacking pad -so always check these regularly
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