Just last week I fixed up a yellowed Keithley 2400 front panel using this technique. Worked like a charm. This was not my first time. I've done maybe 10 front panels of various gear now. I use "40 vol" (very high strength) gelled hair 'bleach' (peroxide). I do not add anything such as oxy clean (some say this is a needed activator, not in my experience).
Here's what I do:
- put on eye protection and a pair of Nitrile gloves. Or two pair, even better. You do not want this stuff getting onto your skin or eyes. It is strong enough to badly burn.
- wash the part very well with soap and water then rinse and dry. Oily surface will prevent the water-based peroxide from working.
- Paint the gel onto the surface of the plastic, trying to keep it on the problem areas (i.e. avoid display lens, etc.)
- place part into a glass or ceramic dish with high enough lip to clear the item.
- cover with one layer of plastic food wrap to prevent evaporation.
- place in full mid-day sun
- rotate every 5 minutes to get even UV exposure.
- After literally only 20 to 30 minutes (for light/moderate yellowing) in the sun, the job is done. It should not take hours and hours. If it does, you have ineffective chemicals, so find something better. For very deeply yellowed stuff, re-apply gel every 20 minutes, and don't expect a miracle, it may never get there.
- Rinse extremely well under running tap water for several minutes. Use a brush to ensure you get into all the nooks and crannies.
- Blow dry with compressed air - the gel won't blow off as easily as water so you will see if you missed some.
I have found that the silkscreen on the Keithley stuff is not harmed after just 20 minutes. However, when experimenting with materials and methods years ago, I did have one soaking for many hours, and when I rinsed it, the silkscreen ink had softened enough to rub off under my fingers. D'oh. Be careful.
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