Tez at classic-computers.org.nz brings the bad news that the answer is: maybe not. He has the details in his post, but the pictures speak for themselves. These computers were Retrobrighted then stored in the dark but the yellow still returned. There's several more examples if you click.
Before and after is the familiar success story. Before, just after and long after is another matter, for him.
So, Retrobrighted ABS can go yellow again without further exposure to ultraviolet light. But there is one possible upside here: these computers were stored in warm conditions. Perhaps heat is something to look out for as well as staying out of the sun?
Tez has some theories to explain his findings:
So keep an eye on anything you've Retrobrighted and maybe take some pictures to see if this is widespread. I haven't tried Retrobrighting anything yet myself.The original damage from light causes degraded or free bromides throughout the case from the fire retardants. Retr0Bright only takes these away from the surface layer. However these bromides can migrate fairly freely through the polymer. They don't need light to do this. Migration is probably accelerated in hot conditions. These pre-existing bromines from the original damage migrate to the top and within a few years the surface is yellowed again. The rate of the regression depends on the inital extent of yellowing (the more yellowed, the more bromides down deep), heat exposure and the nature/quality of the plastics.
If this hypothesis is correct then a UV sealant will not protect the case. Or any sealant maybe. Unless pre-formed bromides from previous light damage can be stopped from migrating to the surface (and I don't know how you would do that) Retr0Bright is only ever going to be temporary.
Bear in mind this is just a hypothesis which fits the evidence but has not been proven. To prove or disprove it would require research. It seems a logical supposition though.