Unprotected exposed metal is going to rust or corrode, unless the panel is stainless steel or gold.
As for some kind of rust prevention, skipping on paint or primer depends on the product you use. Unless the clear coat specifically said it will work on un-primered bare metal you need at least a primer coat. And yes there are products that do work on bare metal, I don’t remember the actual name of the product but Rust-o-leum and Krylon both make clear coat sprays for bare metal, you have to read the can of clear coat very carefully before purchase.
I also do highly recommend sanding, as it will smooth out the surface, gets rid of any oxidation on the metal and create a surface for the clear coat to ‘bite’ into the metal and have a proper chemical bond. You want the clear coat to bond to the metal so that the clear coat do not chip, flake or peel easily and it be harder to wear off. Some people will primer the metal, add a matte color paint like gray then clear coat it as the combination of primer, paint and clear coat offer more protection that just clear coat alone.
How many coats depends on your product, you and the surface in question. For a Top panel no less than 3 coats, but you want to go with at least 5 and 8 would be overkill. Keep in mind regular play can and will wear off the clear coat, and regardless of how many layers there are it will happen eventually, it could be years but it will happen.
Keep in mind some people will take the extra step off adding a acrylic, plexy or lexan piece over the metal any ways. This is to offer the metal a extra layer of protection that clear coat will not. The Plastic will offer a protective layer that isn’t going to wear though. This does not mean you should skip the clear coat.
The Plexy is not to prevent oxidation but to offer protection from the clear coat from wearing off.
You are not going to get a mirror polish from sanding alone. You actually have to buff the material to get a mirror shine.