>>11318238When I clean bulk parts I use hot water, regular dish soap, and a very tiny amount of white vinegar. If I'm going to sun dry the parts I rinse with distilled water so it doesn't leave mineral spots. This is less if a problem if you're going to towel dry the parts.
I would simply accept whatever yellowing your pieces have undergone. Yellowing is an oxidization process, like rust. It relies in there being new material exposed to continue the process, so it will settle at a point and not continue unless you increase the sunlight/oxygen exposure or reveal new surfaces. Hydrogen peroxide re-opens all the microscopic pores in the material and - yes - whitens them, but it also reveals significantly more surface area on a microscopic level so the I'll yellowing comes back SIGNIFICANTLY worse. Worse than if you left it alone.
I did an experiment a few years ago to prove this point. On the bottom brick, the LEFT side was treated with hydrogen peroxide until it was as white as the top brick. It was then left in the sunlight for approximately one year, and now you can see that despite sharing the same exact exposure level, the treated side yellowed FAR worse than if I didn't touch it. It sucks but you just gotta deal with it.
