Hi.
I’ve been cleaning a poly tunnel today that I’ve been asked to make use of at our allotment. It’s large, 20 x 13ft and though quite old and been neglected for a while but the plastic seems really strong. Just used cold soapy water with a soft hand brush and a long handled soft brush for the higher parts. A light swill off afterwards seemed to freshen it up a treat. Good luck with yours.
