You really should be using sanding discs like the ones below either 24 or 36 grit. As stated, you have to be careful not to take off the parent metal. These discs allow you good control and work great. Right angle grinder/sander and you vary rpm as necessary and always keep it moving.
You use backing pads like the ones shown below. As the disc wears, you go to a smaller backing disc and trim the edges of the disc with a pair of tin snips - does not need to be perfect.
https://www.autobodytoolmart.com/product/3-pc-phenolic-backing-plate-setSanding DiscsThat is the technique I used in this guard on the edges - there is no plastic in that corner.
