OK, another day, another update...
The top half of the case got a day in the peroxide tank, even though it was white to begin with.
The bottom half got it's 2nd day in the tank.
The top pic is how it looked when I bought it, the bottom pic is this evening. The top half didn't get more yellow, it's just the white balance on the iPhone. The key thing is both halves are now the same color (white):
The key caps have now also had 2 days in the peroxide tank.
The top photo is how they looked when I bought the machine, the middle photo is after 1 day in the tank (3% peroxide and Oxyclean), the bottom photo is after the 2nd day in the tank (7% peroxide and Oxyclean):
I need to put the keyboard casing in the tank one more day to try and even out the grey. Also, it's hard to see it but the keys are still a little grungy and uneven, so they also need another day.
But I'm out of fluid so whether or not I continue tomorrow depends on whether or not I can find any locally.
If not, I'm out of time this week and will just reassemble as I have other commitments, but overall I'm very pleased.
I'm not sure if the OxyClean helps at all, or whether the layer of foam it creates on the fluid surface is detrimental as maybe it blocks the UV light?
I'd say buy 7% peroxide from wherever is cheapest by the gallon and mix 1:1 with water.
You'll need at least a 23" x 16" x 6" container (I paid $10 for a clear Sterilite one), and 3.0-3.5 gallons of the fluid to do the case, the remaining half gallon can do the keyboard keys in a separate container. You'll need another 1-2 gallons to do the keyboard casing.
This is for each session. I don't know if the fluid can work for several days, but I used fresh fluid each day and you'll take at least 2 days.
The stuff I used cost me over $8 per gallon from Amazon inc delivery (should be cheaper if you're not paying shipping as it's heavy?). I watered down the 7% fluid by 50% for the case (but full strength for keyboard) so 6 gallons of 7% cost me about $50 and lasted 2 days. I'll need another gallon or two to finish off the keyboard casing due to my mistake of not fully covered by it.
A blue sky with some clouds seems to stop the whole thing heating up too much, I wouldn't do it in hot sun.
It seemed to work fairly well in overcast conditions, but slower (wastes the fluid a bit maybe?).
Make sure the whole thing is fully submerged to avoid streaking or blooming. Weigh it down with water filled glass on top if need be.
Stir the keys in the container every half hour.
I also set the containers down on reflective kitchen foil, and put a wall behind and to the side of them of the same.
Wear gloves. The stuff does burn albeit gently.
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