Use hot candle wax.
You will need the pot hot enough to melt the wax, and an oven roasting tin to keep the wax and pot in at just on 100 degrees C
The pot will soak it up like a sponge and become slightly darker.
try cooking oil but it is going to have that greasy feel. If not varnish, oil paint like Rustoleum or Tremclad. Then if you are doing it only on the inside, plastic wrap.
If you want it to be WATERPROOF then you will have to have it fired and glazed... even varnish won't make it waterproof. If you mean a glaze of some kind, then wax may help
engine oil
may not work but candle wax rubbed all over it..
How can i waterproof a terracotta pot at home with daily things at home? Not varnish
