I've gone down as low as 625 grit sandpaper, using just edges of the paper and small sections just pressing using my fingertip to get all the gunk off. If there are some deep areas, 625 grit, 1k, 2k, 3k, and a final polish using 4k sandpaper.

Try using the lower grit, use your index finger on a small section of paper, and attack each of the spots until they are gone, then go up the progression. I took a De Pews razor to mirror finish by doing that, with lots of soaks in between. I think the Evapo-Rust, since it basically is safe for scales and only going after rust, is by nature going to be a slow process.