So far I have gotten two batches of steel tube. The first was ASTM A500 and the second was ASTM A513. Each was all they had in stock for 1x1x0.065 tube at the time. So what's the difference, besides about 13 whatchamadingers? In terms of mechanical properties, not enough for me to worry about. They're both pretty standard hot rolled, seam welded steel tube. However, at least as stocked by my steel supplier, the A500 was protected by a light mill scale...Where as the A513 was cleaned and protected by an oily residue. Then which is better? During initial handling, neither is great. The A500 leaves black grime on everything it touches, and the A513 leaves oily residue on anything it touches. However, the oily residue cleans off easily with a quick wipe down of Acetone. The same cannot be said of the mill scale.
One of the popular ways to rid yourself of the Mill Scale plague, is by using a diluted acid solution, which can cause all manner of hazardous material problems that I have absolutely no interest in dealing with. To mechanically remove it though simply takes time...Much more than anybody wants to spend doing it. Some people simply do a half-assed cleaning of the tube, while only doing a whole-assed cleaning where the welds will be. The idea is that the mill scale is well adhered to the tube, and there are paints that at least adhere well enough to the mill scale for it to not be a problem...But if you couldn't tell from my previous tube prep on the A513, I'll be doing it the complete and total assed way.
As seen in the picture below, at the top is the original flat black mill scale wiped of the initial loose grime. Below that is the 'half-assed' cleanup. This is a few lighter passes with a wire wheel, and has basically polished the mill scale to kind of a satin black. The bottom shows the fully cleaned up steel, after numerous heavier passes with the wire wheel. Off to the right, you can see the pile of tubes in the semi-finished state, and the one tube on the left that is fully-finished.
So if your steel supplier tries to sell you on tube with mill scale...In the immortal words of Bing Crosby, "don't think, don't pause, don't hesitate for a moment, just spit in his eye."