what grits did you use ?I called Heatsinkusa about the flatness and I have to say they were great about talking to me. They use a industry standard for tolerance of flatness and for mine that was .014 thousandths,mine measured at .012 to .013 so they were within tolerance. They actually did a spot check of their stock and said most of it was .008. They told me that each profile is different for it's tolerance depending on the difficulty of extruding it and it can fluctuate even in the same run. It looked to me that this flattening by hand was going to be time consuming so I solved that with the purchase of a disc sannder. It looks like over kill but I got a 12" disc sander so that if I used a larger profile in the future it could handle it. Got it on sale for $120 and I think it was money well spent,took maybe 30 minutes to flatten 8' of 3.5" heatsink and that includes setting it up and putting it away. Now all I have to do is touch them up were the COB's go. I wasn't willing to tighten down such a large COB as the 3590 with that much of a concave surface,especially since I turned the holder so the screw holes fell in between the fins so that the corners of the COB pointed the the sides of the heatsink.
http://www.harborfreight.com/12-inch-direct-drive-bench-top-disc-sander-43468.html
gotcha. I picked up an orbital sander and a rubber block been wondering what grit I should start with and end with.It came with 120 so I just flattened with that but I'm trying to find something finer locally but I might have to order it,still a work in progress.