Yesterday I found myself cleaning up a bit of square tube in preparation for welding and later paint. It was outside, but in SW Arizona, that just means it has developed a small amount of protective rust over the mill scale.
I started with an angle grinder and a flap disc. It made short work of the light loose rust, but would hardly touch the mill scale. It also showed some rust down "in the" the mill scale. I probably could have stopped there, but I got carried away with myself. This stuff welds really nice if you have bright shiny metal and takes a good paint coat as well with an etching primer, and proper cure times.
I probably I could have just used extend or some other rust neutralizing primer after knocking off the loose rust, but this construct will be going into a humid environment.
I noticed a fresh flap disc stripped rust, and mill scale amazingly fast. It did more work with less effort in the first minute or two than it did in the next ten. Now the flap disc was still nearly new, but it was performing like one that was half worn away. There was still plenty of grit, but the edge was mostly the flaps rather than the grit, and the face was the same, but also gummed up with mill scale.
I am NOT trying to get longer life out of a flap disc. I am trying to get more time performing like the first minute or two of a new disc. I don't care if the total life of the disc is less. Is there a practical way to dress these so that you can expose more grit quickly. I tried spinning it against a wire wheel on a bench grinder. The result was less than amazing.
Sans the solution I am looking for is there a better disc for stripping metal than an abbrasive flap disc? I can do the work with a grinding wheel, but its much more skilled to avoid gouging the base metal and doing damage. I tend to only use a grinding wheel to make the bevels before welding.