if it's anodised then yeah, caustic soda is the one to go for.
Add the caustic soda to some warm/hot water (the hotter the water, the more powerful it is). I think a toothbrush works well for frames and doesn't melt like paint brushes do. try scrubbing a bit of the frame first, leave it on then wipe off with cold water, this will give you an idea how fast it will work, then go from there really. The frame will look pretty dark at first, but if you wipe it down afterwards and maybe use some autosol that will get it up to a shinier finish.