Adhesive

When it comes to making crafty containers or sticking [[Magnets |magnets] to cache containers, adhesives become important.

A geocacher contacted Selleys for their recommended product for poly plastics, and was advised that "none of the Selleys adhesives will adhere to Polypropylene or Polyethylene". Check their individual product data sheets on line if you're not sure.

Many of them are also not water resistant in outdoors situations. Selleys actually referred them to Loctite for a suitable product.

Loctite 770 Activator/Polyolefin Primer is designed for "difficult to bond substrates which include polyethylene, polypropylene, polytetrafluoroethylene (PTFE) and thermoplastic rubber materials". You can read more about it, and suitable adhesives to go with it, here. 

Loctite 406 seems to give the strongest bond with polypropylene, which is what most plasic cache containers, such as Sistema, are made from. Loctite The stuff is a bit pricy though.