Wickes High Performance Acrylic Waterproofer
http://www.wickes.co.uk/invt/240148

It works very well (not just by reviews there). The only thing is
whether it is UV proof if you used on the sunshine side - rather than
underneath.


Frankly it sounds like the panels are defective so out of warranty or
not I would investigate further (are there sufficient supports as
specified by the corrugated panel manufacturer).