Rusted Sentinel has one ability which causes it to enter the battlefield tapped. This is a static ability that generates a replacement effect that is applied to the action of putting the Sentinel into play. (This will most often happen as the Rusted Sentinel spell resolves.) The Sentinel does not enter the battlefield and then become tapped. Because of this, effects that remove the abilities from a creature will not change how the Sentinel enters the battlefield. For example, a player cannot use Turn to Frog to change what happens and somehow cause the Sentinel to enter play untapped.