I figured it all out. I've been doing a lot of Retro-Brite on ST's and many other cases. The all come out perfect.
Here is what seems to work best and why.
Get something called 'Developer' from a beauty supply store. They come in many sizes and strengths. I suggest 30 strength. Next, get a clear plastic storage container from any store like Walmart. Make sure it will cover whatever you are Briting. Then get a 4" cheap paint roller like thishttps://www.lowes.com/pd/SHUR-LINE-Poly ... n/50019300
The tight, low nap works best. While getting the roller, get some latex gloves, like dish washing gloves, not the one use type, as you will putting on and off several times.
Take your system apart. Wash the case parts you need to Brite. Dry them off. Get a coffee mug of hot water.
Go outside with all your stuff above. Find a good patch of cement in the sun to work on. Get a garden hose, and water down that cement. Put on some gloves, and slather on the Developer. This is the good part that makes it all work. Use the roller to put a nice even coating of the developer over the case. Move the roller slowly, and evenly. Put the case on the wet cement. Fill the coffee cup with water, warm water is best. Put it behind the case. Cover both with the plastic storage box.
Wait about 25 minutes. Then check your work. If it needs more time, give it another 25 minutes. Then check again. Oh wear the gloves if you need to move the case to stay with the sun. If on this time it still needs more Briting, slather on another coat of developer, and repeat roller, & wet cement process.
What is happening? Well the wet cement, and coffee mug keep the interior of the plastic storage case very wet, and humid as the sun heats it up quickly. This way the Developer dose not dry out. If you look at my first post in this thread, you see what happens when the Developer drys out. You get that white discoloration. After the first 25 minutes, when you look at the case, the Developer white color is gone, and there is a very even clear, sheen all over the plastic. This is what you want.
Now for some tips. On ST's, there is the vent on top, and the crease that follows the top of the F keys across the case. Use a paint brush first to get Developer in the groves, and the rest of the case for that mater. Use the brush to smooth things out in the grooves as best you can. The use the brush to get the inside edges of the keyboard areas, or any other holes. Use the roller last!
Depending on the heat of the day, you may need to rewet the cement after all this brush and roller time.
I have done this in the morning mid-day, and late afternoon, and get the same results. Clouds do not seem to slow things down, either. I am in the northern California latitudes. Things might be a bit different where you are. Temperature dose not seem to matter. I have done this on 100' and 60' days. I get the same results. All cases come out perfect.
The amount if time all depends on how yellow, and what kind of plastic you are dealing with. I had one case that took 3 hours, and several re-applications, others done in just 20 minutes.
I do this with keyboards, 3.5" floppy drive face plates, and eject buttons. I don't use the roller, just the paint brush, but I keep them in a very moist environment.
I see that a lot of videos, and forum posts say to use clear plastic wrap over the case. They are trying to do the same thing as my large storage contain dose. Hold in the moisture. But wrapping is uneven. The wrap squeezes out Developer, and pushes it elsewhere. So there very little, if any, in some places, and too much in others. They often get the same results as that picture above I mentioned.
Now the cleanup. After a quick rinse in a sink. I use a wet dish cleaning brush to do a good scrub, and re-rinse. You want to get all the used Developer off. Use a towel to dry things off, then let them air dry completely.
If you do not want them to get yellow again for a very long time. Spray them with a clear lacquer or varnish of your choice. 2 or 3 light coats are much better then 1 heavy coat.
Then Happy Retro Computing !!
PS I should probably make this it's own thread.
UPDATED: Falcon 030(FX-1)(CF int & removable SD), MSTE 4+PS3000 4160STE 2.06+HDFD STacy4-internal-USD 1040STFM+I.B.Driver-5.25"FDD 1040STF 1040ST 520ST-AdSpeed 400-48k 800+810x2+820+822+825+830+835+850, 800XL+IndusGT 600XL 130XE+XF551x2 Portfolio 1200XL APE(Warp+32in1 OS SuperVideo 2.1 256k RAMBO)+1050x2 USD'd+SIO2PC, PC1+PCH204+PCM124+PCF554x2