Elbows usually go bad from the inside - either rotting or coking up (usually both). So an outside coating would only make it look better, not last longer. A friend says he dips his Volvo
cast elbows in Por-15 when new and they last longer. I simply had new ones made out of stainless when the old ones went bad. The stainless fab cost half the price
of the new cast elbows from Volvo.