Like most people I bought these to assemble furniture and am very happy with them. They're exactly the same as the ones in the Home Depot plumbing department, but cheaper. All black tubes come with a greasy coating when purchased that will rub off and leave marks on the surface, but the coating on these turned out to be a little lighter than the ones I bought from Home Depot, which was nice. To use it in my projects I hit it with sandpaper or steel wool and covered it with matte black spray paint (don't forget to glue the threads if you do this). I can't comment on the effectiveness of tubing in plumbing applications. But as far as I can tell, they're almost identical to the pipes sold in the plumbing department, so in theory they should work fine.
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