I used a fine metal polish called Simichrome. I cut the head off a long m4 bolt and put it in a cordless drill, applied the cream with a soft cloth while spinning it
The yyf lowerkase is awesome! If I remember correctly those came out during the 2021 A-May-Zing sale they ran that year. They are made in the usa and retailed for $30 I think due to them all having some vibe. Same shape and size as the yyf 44 and 444 (hubstack version) but made in all stainless steel.
The first iteration released during a-may-zing sale week was the Sprite. I believe it was the same CAD file but in aluminum, and it was super lightweight at around 24g. Then they tried it with ss and got the Lowerkase.