Tub? Was it a completely transparent tub? And was the tub made of something that blocks UV light?

I did my deyellowing in the Spring, when UV light intensity is the greatest. It's also a nice time of year because you get a lot of sun but not a lot of heat. Heat will dry the gel faster.

I also used large sections of whitewashed cardboard beneath and around the items being deyellowed, so as to direct even more UV light to them. You can see that white cardboard in my photo here:




I did the same when I deyellowed small items in the liquid form of RetroBright, as shown here:




But I believe the key to my success was frequent reapplication. Again, I did that once an hour. Moreover, I remade new batches of the gel once per hour too! I did not make one big batch at the beginning of the day and use that all day. Another benefit of making multiple small batches is heat. I had to use heat to get the gel to be a gel. And the heat accelerated the whitening action, above and beyond a cold gel. You can read more about that in the description I wrote underneath the following photo: