Bloody seatpost ain't shifting
this should work, never failed me in 30 years !!!!!
if you are going to repaint the frame then warm the frame around the seat post and down the seat tube with either a hair dryer or a hot air gun but it may start to melt the paint, the steel will expand before the alloy and then spray WD 40 down between them it will smoke to start with but then will seep between them. put an old saddle on to use to force the post round, you might have to do it again and again.
if you do not want to damage the paint, just warm the seat post itself and it will expand and warm the seat post too again fill with WD40/Plusgas.
may take a few attempts.
if its really struck its an oxy/acet job and a respray thats how frame builders get them out...