Applying Rust Bullet to porcelain is generally not recommended. Porcelain is a very smooth, non-porous material, which means that coatings like Rust Bullet will have difficulty adhering to its surface, leading to peeling, flaking, or chipping of the finish over time.
If you're looking to protect or alter the appearance of porcelain, it's better to use products specifically designed for use on ceramic and porcelain surfaces. These might include certain types of ceramic sealants or coatings that are made to bond with glossy, non-porous materials.