Once upon a time I added "bedrails" on my pickup using the 'liner in a can' stuff.
My reaction is mixed. Initially the look and results seemed really good. However, my fatal mistake was to not completely cover all areas in primer first. The instructions (incorrectly) said to spray right on top of paint. Certain areas just flaked away. Sad.
I purchased this product at Canadian Tire...I can't recall the exact name. There were 2 to choose from, the other had an elephant or rhino on the can. Again, my memory fails me.
Previous to this I had a friend (who was in autobody) fix up significant paint damage to my tailgate with this type of 'in a can' liner and it lasted 2 years before going slightly gray. It didn't fade beyond that. He must have used a better product, because the 'bedrails' started going grey much sooner.
I think it would probably work on something like your westin... you can wait until rust is a problem, then consider this or go professional (line-x or whatever). Personally I would go professional.
J