Hi,

Plastics and plexiglass, as well as rubbers and vinyls, are made with petroleum.  If they are manufactured with a high amount of petroleum, the glue cannot get a secure or tight bond since the surface is too slick and oily.  A lower amount when manufactured, and the glue will work just fine.  Since there are over 20,000 manufacturers of these types of materials, it is almost impossible for us to know if the glue will or will not work so basically it becomes a trial and error application.  This is why we usually recommend contacting the manufacturer of your outdoor greenhouse, for example, and ask them for a suggestion as to what type of adhesive to use since they are familiar with the types of materials being used.  

But I will certainly pass your suggestion along and perhaps we can be clearer in explaining this to others.  Thank you for your suggestion!

Mary-Ellen