But there's a fair number of collectable sets with ABS cases out there (many 1950's & 60's mantle radios come to mind), it's not only white/light-coloured ABS that suffers from bromine yellowing, and cigarette tar is actually fairly easy to clean off*.
Have a look at these before-and-after photos of a grey 1955/56 AWA 565MA I've been working on:
Originally the whole case suffered from substantial bromine yellowing; these pictures are after both halves have been cleaned several times of dirt, dust, tar, and old wax/polish, and the rear case has been treated twice with Retr0bright. Note the relatively unaffected rectangle between the feet on the front half of the case - that's where these sets have a metal mounting/strengthening plate fitted underneath…
It's also worth noting that the Retr0bright had no effect on the cream fascia**. I'm not sure what sort of plastic it is; possibly a hard styrene. I have seen the cross-section of a broken fascia and it was the same colour all the way through, indicating that it was probably cream to begin with and has "UV-yellowed" only slightly (as can be seen around the holes for the knobs). The darkening of the fascia on either side of the dial in that picture is heat damage from the dial lamps.
Convinced yet? I hope so. At the very least it's worth a try, isn't it?
Aside: it's interesting that the underside of the case was heavily affected, yet the part of the underside clamped under a metal plate was totally unaffected. That - coupled with other reports of plastics yellowing despite being kept in their original sealed plastic bags in total darkness - suggests that it's the presence of oxygen, not UV, that leads to yellowing. In fact the areas on this case that were the most exposed to sunlight were among the least affected by yellowing (although they suffered far more from other sun damage).
The couple of plastics/polymer chemists I've discussed this with theorise that in fact UV tips the oxidation reaction to one that favours something other than the production of bromine oxides - or, in other words & counterintuitively, sunlight slows down the rate of yellowing. But, if that's the case, why is it that the inside of cases is rarely affected? Something to do with the manufacturing process forcing excess bromine-based fire retardant to the outer surface?
Tips for Australians & others:
Hydrogen peroxide can be bought in 30%-50% solution (often with a trace amount of silver nitrate as a bactericide/algicide*) from some - but not all - spa or pool supply shops. This can easily be diluted down with water to the 15% or so needed for Retr0bright. An interesting side-effect if diluted with tapwater is that the silver nitrate is at least partially converted to silver chloride, which is photosensitive & will turn the Retr0bright a nice light violet-grey in use. Luckily for us, it doesn't affect its de-yellowing properties.TAED seems actually to be fairly rare in Australian laundry brighteners. "Vanish Oxy Action" is the usually recommended source, but here it contains no TAED at all. Luckily, the two "gold" varieties - "Vanish Napisan Gold Oxi Action" and "Vanish Gold Crystal White Oxi Action". If you can't get hold of these, check the Material Safety Data Sheets (MSDS) of whatever alternative products are available for ingredients with the CAS No. 10543-57-4 (TAED & its synonyms).
For what it's worth, while I've previously been lucky enough to make Retr0bright using reagent-quality hydrogen peroxide and lab-grade TAED / gylercine / xanthan gum, the solution used for the above pictures consisted of spa peroxide, Vanish Gold Oxy Action, and glycerine and xanthan gum from Coles. If anything, I think it worked a little bit better than the stuff I made in the lab...
* Handy tip: a little liquid laundry detergent - just enough to get the water starting to froth - in a tub of warm water, used with a sponge, will get 90+% of cigarette tar off hard surfaces. Any remaining after that can be removed by the more usual means - a dash of ammonia & white vinegar in water, proper sugar soap containing TSP, etc. - after spot testing. Take it from a smoker - if that can clean walls where the tar deposits were so heavy it actually formed runs, it'll clean it from plastic.
** In fact, it had no real effect on the dial paint either - I tried it on a small section hidden behind the fascia, and all it did was clean the grime off and bring the paint back to a clean cream colour. Even now, several months later, there's no sign of permanent or ongoing damage.
