Principle: The strong neodymium magnets are embedded in a steel pot. The steel pot increases the adhesive force of the magnet on direct contact with a thick iron surface. If there is no direct contact with the counterpart, or if the steel plate is thin, painted or rough, you can strain it much less.
Shear force: The indicated adhesive force was measured perpendicular to the surface to which the magnet should stick. It takes a lot less strength to move the magnet sideways. Therefore, you can strain a pot magnet much less if you use it on a wall as opposed to using it on the ceiling.
Rust: These magnets are not rust-proof and therefore intended for dry indoor use.
All further information regarding these products can be found in our FAQ about pot magnets.