Thanks for posting about the other brands of spoilers out there. I wasn't aware that buying a non-painted one could lead to troubles.
I bought one of those Ebay roof spoilers as well. The one I bought was aready painted with KH3 (black obsidian) and clear coated - it matches the finish of the care perfectly. Mine came from Japan and fits snug and perfect - no issues with the paint or the product after at least a year in the hot FL sun and snow / ice up north. If any of the trouble you mentioned should happen to me, I'll go with Sarona.
My spoiler curves upward and is smaller (about half the size) than the Sarona part but that was the 'look' I wanted. The Sarona spoiler is abolutely great and it's almost like having a spolier and mini visor at the same time, but it was larger than what I wanted.
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