I've been a proponent of Kroil a long time.

Way back in the early 90s, my GF brought home a problem bike. It had been ridden in the snow and ice and salt of Minneapolis for a season a put away wet. The seatpost was frozen in place and had defied 3 owners and 6 bike shops. As a result, she got the thing for free. I had Kroil on hand because I was into vintage motorbikes and keeping rusty old cars alive.

I gave the seatpost a good shot right where the post entered the frame after removing seatpost bolt I left the frame on the porch (Kroil is kinda smelly) and waited about 5 minutes. Then I came outside and with no tools at all, gave the seatpost a little bit of a wiggle and it came out smoothly and still usable despite its prior abuse.

This issue of seatpost removal should never come up IMO. Everyone that works on vintage bikes should have a can of Kroil as nothing works as well. Its the inside tip that master mechanics have.