^ If you have nothing else, try Cola. If you have WD-40, try this.
Both are better than nothing, but not much.
If you really want to loosen screws effectively, take something like rust-shock or chemical wrench, or mix some acetone with hydraulic fluid. Latter one is cheap, but destroys paint, so use only on unpainted parts..
Never going back to WD-40 again, this stuff is almost useless.
@Betonov: greenhouse looks nice!
