rust our biggest enemy

[Ivan141]

Drill small access holes from the top, then spray from inside with your rust inhibitor of choice. Do this on a few spots along the whole length of the sills.
Finish the holes with a bit of paint on a brush to prevent them becoming new rust spot starters (before spraying anything in offcourse). Most of the commercially available coatings come with plastic caps for closing off the holes afterwards.

Take your pick from any type of hollow space coating;
ML Tectyl
Waxoil
Mike Sanders fat (supposedly the best).
etc..

Or just use some old used oil and apply generously. If your repeat this
last one once every couple of years it is supposedly pretty good, and cheap.
Just not too nice on the environment. I do think you would need some drainage for this last method to allow the oil to exit again.