Evaporust and small grit sandpaper is the best....hands down. I restore old tools and nothing works better then this. Let it soak for a day or two and it wipes right off. You can reususe it as well. ://shop.advanceautoparts.com/p/evapo-rust-rust-remover-32-oz-er004/10182522-p?c3nid=10182522-P&c3ch=PLA&c3apidt=28033512410&gclid=CjwKCAiA15vTBRAHEiwA7Snfc2H7tmElZey1YtxtzLX4m_Yptb4q2A5mv0JVcV4LnlLF6azLLt_TMhoC2UwQAvD_BwE&gclsrc=aw.ds
You will not be able to get into where the rust is without a liquid. Evaporust literally will dissolve the rust away from the piece and you can smooth out any pitting and it will look like new. Steel wool will destroy the finish.
Drift the sight out, disassemble, drop into a cup of evaporust for 24-48 Hrs depending on how deep it is.. Take it out, run under cold water, dry and sandpaper and remaining pitting. Oil the parts and reassemble.
Used this method on guns, old tools, Knives, ect. and it works like a charm. Only other thing that comes close is electrolysis but i doubt he wants to go through that mess.