I was told yesterday, when buying a second hand bivi bag at a bushcraft car boot, that if you pop it in the tumble drier it re-energises the coating.
I ran mine under the tap and the water ran off it like a ducks' back so I've not needed to try it.
I used to work for a couple of camping shops and spent time talking to reps etc but I'm afraid that tumble drying to it re-energise the coating is a bit of a myth.
It works on some breathable fabrics
but not Goretex.
I'll try to simplify it for you:
Goretex is a membrane with holes in it's attached to a nylon sheet.
unless you physically make a large hole in it Goretex will always be waterproof (as the holes are too small for water droplets) for many hundreds of people
it seems to leak but this is usually when water vapour is trapped inside(read below).
when Goretex is new has a waxy sort of finish(it is not wax), and water falls of it "like a ducks' back"(as you said).
after time muck, sweat, Grease (from your body) and general use wear away the waxy coating and dirt will clog the tiny holes (so you sweat more and hence get wet inside).
when it get's like this you need to wash off the dirt (with "old school" soap flakes), and re- apply the waxy coating (which you can buy from camping shops).
when you've re-applied the Nikwax/Grangers fluid you can tumble dry the item on a warm setting which helps to "melt" the waxy stuff on.
Other breathables (again not all) are different.
some are like a plastic layer soaked into the nylon this is the one that will get better with tumbledrying. (but it still needs to be cleaned first (with soap flakes). somehow the heating of the fabric changes the molecules back to a previous state(sort of like memory foam).
Oh and
NEVER put wax or silicon spray on breathable fabric it ruins it, you can't get the stuff off again and it clogs all the pores permanently!
use the proper Nikwax/Grangers fluid.
Goretex doesn't work well in a humid enviroment( like a jungle) as the outside needs to be cooler to draw water vapor through the fabric.
Goretex works better on windy days!
Always put something under a bivibag to protect the base from thorns/sharp stones etc.
If you do very hard physical work in Goretex the chances are you will produce more water vapor (warm sweaty air) than it can cope with.
I had to explain this to 2 -3 people every day at the camping shops!
but to answer the question
is Goretex waterproof?
if brand new Most definitley yes, otherwise people wouldn't buy it.
second-hand......depend how careful the last owner was!
