"Ian White, G3SEK" wrote in message ...

Before you do, you'd better read this:
http://www.diyfaq.org.uk/humour.html#foam

For 'canoe', read 'kayak'... and don't say that nobody warned you!

Worth an epic poem!

I particularly liked the bit about the wailing and the hair removal.

Yes, I was already rather concerned about this. The so called
"mono-component" storebought PU foam is sometime better referred to as
"waterblown". It's rated 90% closed cell, which is fine giving the
tiny size of the typical cell and the all-around sealing I am going to
provide anyway.

The problem is, the expansion ratio is hugely unpredictable. It is
said to go up from 8 to over 20 "if water is present", with "present"
being left undefined, and probably undefinable as well, although
"wetting" is recommended.

Moreover, as the unlucky canoe builder found, penetration, bonding,
expansion, and setting go hand in hand, meaning the stuff performs
this remarkable routine:
- penetrate every nook and cranny
- soak up water
- stick to surfaces
- gain some texture and firmness
- arch its back and expand (for waterblown: at some ill defined rate
too!), bending out of whack anything that is not as firm as stone or
steel

I did a test in a small box (translucent, probably polyethylene,
formerly contained a bike tyre repair kit). I put in a couple of
airwound bare copper coils, half inch apart, rinsed it all in water,
and while it was wet I sprayed in some foam (_not_ to the point of
filling it), closed it, and wrapped it tight with wire to provide real
closing power. I could see that the foam had not yet filled the
cavity. The DC resistance between the two coils had dropped from
unmeasurable (2Meg) to 150k.

24h later, resistance was back up to unmeasurable, the box looked
completely filled, nice big round blobs of foam had formed all around
the lid, and all the surfaces of the box had become warped under
pressure. Chances are, the coils had been bent around as well - I
could not check if they were short of a CAT scan, for which I am not
yet equipped.

If I'll do foam my balun box - and it's still a big if - I'll first
fashion and fit a temporary, ~50% perforated lid, to give the foam an
easy way out, and to make it easier for me to remove the excess foam
later on.

I'll be very careful if I ever have to do the same to a canoe.