40vol stuff is awesome. I've tested a couple of kinds out in the sun vs indoors - it does nothing indoors, but does too-fast work outside in sun UV. I'll add the same disclaimer for any drive-by readers that I make about Australian sun and retrobrite here too - 40vol, here, in country australian sun, will nuke a mac's case to white blooming in minutes. It needs to be REALLY diluted to give a good result, so here the submerge method is the only useful one. Test first!

I find yellowing can come back subtly over many years, but only in the same places it was originally yellowed; My guess is it's the leeching of bromine from only the parts of the plastic originally damaged in the past. My IIci/IIcx case top is my benchmark there. Kept in the sun originally, it was heavily yellowed on one side with a far less-yellowed shadow of the monitor on top. It keeps re-yellowing slightly in the same pattern.