The picture shows the before and after of a previously yellowed Fortress Maximus. It’s still with the original owner, but he wanted to restore it to its former glory!
I’ve never tried this myself; however still wanted to share the steps he took to restore his Fort Max:
“You take your figure, place it in a bin of the correct size (clear is best), use a drain board grate inside the bin, fill an inch or two of the bin with H2o2 (Hydrogen Peroxide) I use industrial grade 6%, most stores sell 3% (it works, but the process takes longer), place your figure/figures on the drain grate, wrap in a clear plastic bag over the bin + figures (I tape the end shut to keep it air tight) and place out in direct sunlight/or a UV light. As long as the day is sunny or you have a UV light available. Let it sit and do its job. Check on it until your satisfied with the results. Plastic oxidizes over time and causes that yellow/brown ‘tarnishing’ of plastic toys. H2o2 reverses that effect and “whitens” the plastic, it also removes that tarnishing effect on colored portions without damaging them making the entire toy look cleaner and brighter. I normally compliment the result by wire brushing the screws, tightening joints, replacing stickers to give it an amazing quality to show off the reason you want to use clear bags + bins is due to reaction you want to have on the H2o2. UV light reacts with the H2o2 causing it to become a gas, that gas reacts to the oxidizing process of plastic and gives it that “cleaned” look. I normally use a mild dish soap, luke warm water to clean the toy after said process is completed. The end result is amazing! This whole process can take a few days if conditions/available light are right. The lower the concentration of H2o2 % = the longer it will take to get the results you want. My Fortress Maximus was pretty yellow – took 5 days with 6% concentration. Make sure to wear gloves, eye protection, especially with higher concentrations for safety.”