Claudio,
I am guessing you have the glow head off. Try some fuel in the cylinder to see if it loosens it. If not, you need to remove the cylinder and insert a smooth dowel or something similar to tap on the piston and free it. Removing the cylinder with the piston at TDC may free it. Make sure there is fuel or a penetrating oil on the piston.

It may be varnish, but if you put it away with raw fuel in it, it may be rust. Alcohol in fuel gathers water and nitro forms nitric acid.

If it is like that in the cylinder, it may also be like that inside the crankcase. You need to remove the backplate to check.

Good luck with it, and keep us posted.

Oops, forgot to answer...if it is varnish, a Brillo pad used wet is better than steel wool. Gun cleaner works well also...Hoppes is preferred by many. Outers works too.

George

Edit: Additional thought, if the fuel contains much synthetic, it is probably not varnish.