Rain traps - "avoid tarps/fly-sheets that have been treated against decay"(?)

[JTsupertramp]

Just reading about rain traps in Ray Mears' Essential Bushcraft. He warns "Avoid canvas tarpaulins or tent fly-sheets that have been treated against decay or may be tainted".

What does he mean by that? How does one find out if they've been treated against decay or tainted?

I use a tarp cut out from the ground sheet of an old tent and planned to try a rain trap with it. But am now rather hesitant...

Any elaboration/advice would be great, cheers folks.

[Shewie]

The only stuff I know that's nasty is the UV protectant spray some folk use, not sure about the Nikwax proofing sprays etc.

Usually if you're going to treat a tarp you'd do the outside, it's easy enough to just flip it over and use the underside though. It'll probably be PU coated but I wouldn't be worried about it coming off and tainting the water.
I think I'd be more worried about a cotton fly that's had Thompsons sealer or similar on it.

[JTsupertramp]

Thanks for the feedback man. I suppose it's not a big deal then if I haven't personally treated it with a sealant or the like. Will test the old rain trap method with ease of mind now

[EarlyRiser]

My guess would be that he's referring to mould and mildew treatments.

Tom

[gregorach]

I think the key word there might be canvas. Lord knows what they use for rot-proofing...