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If the burner is made of heavy cast iron, rusting is normal. If it bothers you, try cleaning it with a wire brush and then a light coat of auto manifold or wood stove/BBQ spray paint.
I've had a 40" Propane fired Masterbuilt for less than three years.
The first year was great.
Last year I had to spend a lot of time cleaning the burner and getting rust out of the inside of it.
This year the collar ring around the hole in the bottom of the smoker came off in my hand, it and the bottom of the smoker had completely rusted away.
As far as I can tell, the thing is a piece of junk. They're using low grade steel that lasted less than 3 years.
I suppose a metal shop might be able to replace the bottom (until something else rusts/breaks/falls off), but I suspect I'll be better off with a smoker made by a company that cares a little more about the quality of their products.