Andy Hall wrote:
Thanks for that, I'll look for some. The area is going to have heavy
loads, so I do need something substantial.

Fair enough - use just one trowel in the whole process if you can, i.e.
use a stick or something disposable for mixing it, and mix it in
something which is disposable. You'll have fun cleaning the trowel
later - I wiped off as much as possible, and then let it set and sanded
it off - it was persistent stuff. I'd used a bucket trowel for mixing
and a couple for applying, which was a bad move. Might be cleverer to
find out what the right solvent is, of course.