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That picture of the broken weld looks like a pretty thin stainless steel.
Machine robot weld?

Powder paint won't stick to the hot ss substrate. Burns off.
Like in a burn-off [bake-off] oven that ramps up to about 850 deg F and holds for three hours and ramps back down to ambient temperature.

But 'Frit' will. That's a substitute for porcelain.
But is a powder that is melted onto the substrate.
Think of a Weber Kettle.
Need a 1700 deg F oven, at 750 deg F and above it's called a furnace.

Tacoma Bob has the answer. Too shiney? Why not wrap the exhaust with that heat tape used for turbos? Motor cycle shop.

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