Removing stuck or seized seatpost?

I tried to protect the frame paint with bin bags and fed a short piece of hose pip into the seat tube with a funnel on the end to minimise and spillage. The frame still got covered with caustic soda when pouring out old solution and pouring in new. This dried a thick white colour and I was fearing the worst after it came back post warm water clean but a wipe down with vinegar removed it. Overall I probably didn’t need to worry as much about the paint as only a very small amount was removed where I think a blob of parcially undiluted CS sat on the paint over night. I was making 400ml batches using enough CS until I could feel the heat through the plastic container and marigolds, leaving for a few hours / once over night. I would def use this method again all in prob cost £20 3x 500ml CS, gloves, funnel and no need to respray the frame. 👌