Isn't using Tamiya Compound can solve this?

Tamiya Compound can help you on this,  check it out on Youtube. 

there's higher grit sandpaper, with the highest #2000
after that use tamiya compound or similar hobby use polishing compound, usually 3 types, coarse, fine, finish
to end up a surface as shiny as a new car, you need to use coarse -> fine -> finish

I advice against using car polishing compound, it may deteriorate the plastic

final advice, in the second photo, you don't need to worry about nub marks, as the area will covered in the torso after finish assembling

Hi, legenderycity I am quite curious, can you state the step you sand the part?
Is it file->1000grit->1200grit?

For me, I would do file, 800grit->1000grit->1200grit->2500grit/used sandpaper->compound.
Try this out, I learnt this from an senior which have 15+ years of experience in plastic model.
You have to slowly increase the grit for sanding, not from after file, with rough surface straight jump to 1000grit, it will only waste your sandpaper.
Cheers

Hahaha after 1200 actually can apply compound already, most important is earlier step when the surface still rough, you have to slowly increase grit. With flat top coat, it will be fine.
If you want finer touch then end with 1500, 2000, or 2500 grit.
What I did is what I told you earlier file->800->1000->1200->used sandpaper.
Trust me bro, I learnt from ah pek from ICW. No harm to try

#600 or #800 doesn't make much difference, most important is the step up in grit
either #600 or #800, the next one you use #1000 or #1200, then next #1500

btw this is getting too much off topic, there's a thread specific for asking about modelling skills

plus, this steps have been posted many times before, please read it