If you coat the outside of the existing stump, no matter how thickly or
with what material, the wood can still rot out. Assuming the roots are
still in the ground, the bacteria and fungi can go up into the stump
from the roots, eventually leaving you with a hollow cast of your
stump. If it were up to me, I would get some kind of nice, durable,
metal and concrete stand. If you really must have your old-stump look,
though, I guess I would recommend a totally new stump also, that has
been thoroughly treated on all surfaces, and sunk into concrete in the
ground (with some supporting posts or spikes from underneath).