I've tried using empty cartridges for such things as cement when pointing
between slabs. Seemed the ideal thing. However, I found that you cannot push
wet cement through a nozzle. It just firms up like sand on the seashore does
when you tread on it. So maybe this is what happens to Polyfilla. Also, I
suspect the Polyfilla would set in the nozzle between usages.

Rob Graham