Do you have to apply by knife - warm - cold or how.
I don't remember who gave the tip on the other place, but they said to put some
in a cloth and start to rub. The oils work through the cloth and onto the plastic. I either did not have sufficient patience or their cloth was different to mine. I ended up trying a woven dishcloth/cleaning cloth so the peanut butter squeezed slightly through the holes. Worked a treat.
It's always possible they mistyped and meant to say
on, but doing it that way you seem to be constantly dipping in the jar, and I'm sure your wife wouldn't be happy.