Cast iron skillet kashering

I purchased a restored cast iron skillet on eBay. I am wondering about the kashering process. The seller restored a vintage skillet. That involves cleaning very well, then seasoning with several layers of oil. The oil basically is baked into the surface of the item to provide the nonstick character desired. The auction listing says grapeseed oil was used. Will kashering involve stripping off all of the seasoning that was baked in, then libun in a hot oven before reseasoning on my own. Or can libun be done in its current state, with trust in the seller that he did use grapeseed oil, and not something like, for example lard. Thank you for your help.

Libun would work regardless of what oil was used. Libun will get rid of any bleius that the skillet may have.