SpamHog wrote:

My balun box is small, at most 3/4" deep, all metal contacts are
already coated with thin epoxy, and the transformer wire is PVC coated
too, so a momentary contact with water spray, soon to be soaked up by
the PU, should be OK.

Agreed. However, the PU foams that we buy in DIY stores do tend to soak
up water into the outer layer, especially after the smooth surface has
been cut to trim off any excess.

Some foams are notably better than others in this respect, and it might
be better to look for the special 'closed cell' foams that are
specifically rated for use in for flotation aids.

I'll do another test in a small plastic box containing some wiring, all
pre-wetted. As the foam keeps expanding as it sets it may be a good
idea to avoid closing and bolting the box lid before it cured
completely, or at very least to provide some escape holes. More later.

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!


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek