Because when I happen to leave the application brush unwashed for an
hour, it's still usable, whereas when I yesterday glued a piece of
timber in place it started almost immediately to "grab".

In another situation I drilled a hole for a dowel through both pieces
of wood, glued the dowel and hammered it in. But stupidly I had not
previously located a suitable punch to punch it right through into the
second piece, and by the time I did find one (30 seconds) that dowel
was immovable. I've done this before and it works fine as long as one
does not pause even for a second whilst hammering it through.

This is Evo-Stik Weatherproof Wood Adhesive, by the way.

I fill the hole in the outer piece with another dowel cut off flush,
then planed or sanded, by the way(2).

MM