My convertible has noise from moving parts. It's currently coated with another brand's silicone spray—would Bell Hammer Spray still be effective if applied over it?

A certain level of effectiveness can be expected from the use of a layer of bell hammer spray.

However, to ensure the lubricating performance of this product, we recommend wiping off the silicone component before use.
If a strong silicone film remains on the metal surface, the lubricating component of Bellhammers may not be able to adhere to the metal surface.

Points to keep in mind when using Bell Hammer
If there is any residue of the existing spray on the moving parts, wipe it off lightly with a parts cleaner or the like.
After that, spray an appropriate amount of Bell Hammer Spray on the part where the noise is bothering you, and move it several times to blend it in.