retrobrite isn't really a product... there are several recipes now for it. I recommend using a coagulant to make it thicker. People are also saying they are having good success just using the 40 vol bleach. However, the other stuff makes it easier to apply and keep it there. The problem with sun is that it moves and is not super consistent. Shadows and clouds and dust and stuff get in the way. I've tried using a blacklight bulb to do it as well... which is good because it's entirely consistent but bad because the UV output is not as strong as the sun's energy. So if you do it with bulbs I would recommend leaving it for like 24-48 hours.
Ultimately you may need to do many applications of the retrobrite to get any kind of acceptable response on the new astro city speaker housings. Another thing I've seen done is people do paint them.
I just left mine unrestored... I don't mind the look too much once they're yellowed.