Take care when buying shotgun gun slips. Unlined, canvas models can scratch barrels and stocks as the canvas tends to be stiff rather than supple.
Try purchasing lined slips or if wanting to purchase a canvas model, place the shotgun in a gun sock before putting into the gunslip.
The majority of re-blueing I do throughout the year is damage caused by gun sleeves or leaving shotguns in slips when exposed to wet weather.
The Sporting Lodge