Case Polishing

[_Garak_]

I'm in the process of repairing and refurbishing a 1953 Murphy A192



I assume the case is Bakelite with the front panel finished in a sprayed satin ivory / cream colour.

The Bakelite has developed a whitish bloom, especially the top surface.

In the past when dealing with brown Bakelite I break out my tin of liquid Brasso and after a bit of hard work the Bakelite looks like it has just come from the factory.

This Murphy's case is another matter altogether. After four hours of polishing the top of the case it doesn't look much better apart from the surface looking a bit shinier but the underlying bloom is still there. Normally when cleaning Bakelite I notice some transfer of brown colour onto the cleaning cloth, but with this case only the colour of the Brasso can be seen on the cloth.

Could this case be an early type of plastic and not Bakelite? if not can anyone recommend a product better than Brasso to bring this case back?

Thanks for any help and recommendations.

[stevehertz]

on Bakelite I use Greygate polishing compound number 5, and it does an excellent job. I then use Brasso, and then finally shoe polish of the requisite colour.

[G6Tanuki]

I don't really do much involving Bakelite [I'm more into military metal-cased radios] but a while back I re-worked a WWII-era "Microphone, Hand No.8" - see here -

https://www.vintage-radio.net/forum/...ad.php?t=99220

that had gone a bit whitish on the surface.

"Solvol Autosol" and a bit of vigorous rubbing removed the discolouration.

[David G4EBT]

It went into production in March 1953, so still in the Bakelite 'era', but I suspect it might be thermosetting plastic.

Quick way to find out is to apply the tip of a soldering iron in an unobtrusive place such as inside the cabinet. If it melts, it's plastic and the bloom could well be due to UV damage from sunlight, especially considering that it's on the top of the cabinet - the most exposed area. If so, I doubt much can be done about it.

Good luck with it.

[stitch1]

I don’t have this model but have several from the same era and agree it’s probably not Bakelite. You can usually tell if it’s quite thin and flexible.

Rather than abrasive methods you’d use on Bakelite try applying a polish, allow to dry and buffing it. Beeswax (available as a spray too) or a high gloss car polish.

John

[Paul_RK]

I had one, I'm surprised I can't find it now as I rather liked it and I don't think I would have parted with it when half my radio colony went for sale in the North East. My fallible memory says that it was bakelite and that I would have polished it using the usual Bake-O-Bryte a.k.a. Greygate Paste no.5. Certainly wasn't thin or flexible!

Paul

[Goldie99]

This might help:

https://www.vintage-radio.net/forum/...ad.php?t=57257

[stitch1]

Certainly looks like Bakelite on those pictures.

John

[Nickthedentist]

I would say this is Bakelite, and I suspect it has been affected to long-term exposure to sunlight.

Our school science department had a dozen 300-series phones re-purposed as an internal intercom, and after 30-odd years in the sun, many of those had gone very pale.

You might be able to cheat by using woodstain, and/or boot polish.

[_Garak_]

Originally Posted by David G4EBT

It went into production in March 1953, so still in the Bakelite 'era', but I suspect it might be thermosetting plastic.

Quick way to find out is to apply the tip of a soldering iron in an unobtrusive place such as inside the cabinet. If it melts, it's plastic.

Thanks, I used the soldering iron tip and the material did not melt so it's Bakelite and after more hard work with the liquid Brasso the finish seems to be getting better.

After replacing the waxy and Hunts caps and a few resistors the set now works well but sounds very 'tinny'. Due to the set's small size and the central location of the dial glass only a very small speaker was able to be installed (5" with a 4 1/4" cone) plus as this was Murphy's cheapest set no external tone control was utilised.

I also noticed after purchasing the replaced components that Murphy had printed the wrong value for one of the resistors I had purchased for replacement. The value quoted in the component list was (R27) 47Ω 20% 1W which I ordered (I chose a 2W). Afterwards I noticed a note on the schematic which gave an updated value of (R27) 39Ω 10% 1.5W. The installed 47Ω seems to be OK but I will keep my eye on it or should I swap it out for a 39Ω one?

[ajgriff]

Originally Posted by _Garak_

.... so it's Bakelite and after more hard work with the liquid Brasso the finish seems to be getting better.

You need to use something a bit more aggressive like Greygate's compound or cerium oxide as used for glass polishing.

Alan

[Cathovisor]

There is always the possibility that the 'bloom' is actually the filler material showing through.

[ajgriff]

Originally Posted by Cathovisor

There is always the possibility that the 'bloom' is actually the filler material showing through.

My understanding is that bakelite is a contiguous material formed by heating polyoxybenzylmethylenglycolanhydride mixed with filler under pressure. In other words there is no outer layer as such. If I'm right the idea of 'filler showing through' is a false concept. Any bloom is caused by surface ageing or UV light deterioration which should polish out using appropriate compounds. Happy to be corrected if I'm mistaken of course.

Alan

[David G4EBT]

Originally Posted by ajgriff

My understanding is that Bakelite is a contiguous material formed by heating polyoxybenzylmethylenglycolanhydride mixed with filler under pressure. In other words there is no outer layer as such. If I'm right the idea of 'filler showing through' is a false concept. Any bloom is caused by surface ageing or UV light deterioration which should polish out using appropriate compounds. Happy to be corrected if I'm mistaken of course.

Alan

I think you're probably correct Alan.

As radio restorers we tend to think that damage or discolouration on Bakelite can be polished out with Greygates paste, T Cut, Brasso, whatever.

I can only contrast my experience of polishing acrylic pens tuned on the lathe.

True, it's not Bakelite, but the finish 'off the tool' is matt, and it has to get from that state, to one where the finish is so smooth and glossy that there are no scratches which are visible to the naked eye. To achieve that objective, polishing pads of progressive fineness are used, starting with 1,500g, then 1,800, 2,400, 3,200, 3,600, 4,000, 6,000, 8,000, 12,000. None of the stages can be skipped and anything less than 12,000g will leave discernible scratches.

Only then can the acrylic be polished.

Of course, doing that on a lathe at 2,000 RPM doesn't call for any 'elbow grease', but to polish out blemishes Bakelite by hand going through the grades of wet and dry is quite another thing.

https://www.amazon.co.uk/Sandpaper-S.../dp/B07ZFVZ53W

I guess the only way of finding whether surface blemishes such as bloom could be removed would be by stripping the surface maybe with 600, 800 1000, 1,200g the progressing to finer grades would be to try it in on a piece of scrap Bakelite.

I think I'd sooner spray the cabinet!

[Cathovisor]

Likewise I'm more than happy to be proven wrong as there's a lot of "Internet Wisdom" out there that suggests it's the filler showing through (along with dire warnings about asbestos); my understanding was that when Bakelite is pressed, the resin tends to flow out towards the edges of the mould and forms a thin pure layer which is easily polished. I'm sure I've repaired a Murphy cabinet with progressive sanding after gluing with Loctite 406 and reinforcing the inside with fibreglass tissue and resin though; and anybody who's ever polished car headlights will know the stages they go through from matte to clear. (@mods - is this allowed?)

As has been suggested, maybe I need to find a dead cabinet to try.

[music-centre]

My experience is that if the Bakelite is damaged due to long term exposure to sunlight it never polishes up to the very shiny finish that you can get if its in good condition.

In the past I have had Bakelite cases that are dull, especially on the top, and whatever I tried they could be improved but never come up with that glossy finish that Bakelite can have.

If its in good condition it seems to come up well with just metal polish but with some cases that are dull I have attacked them with more abrasive things such as car paint rubbing compound/Tcut ect and however many times you repeat it they never come up really well. Sometimes I have had 2 examples of the same model where one polishes up well and the other never really does.

Best I have been able to do on the dull ones is use the Greygate polish and then shoe polish or the darkest colour of the scratch cover polish.
Steve.

[stevehertz]

I also think that Bakelite varies in the 'quality' of its surface finish. Some seem incredibly smooth whilst others have a slightly grainy finish, and not necessarily as result of exposure to sunlight. Maybe the quality of finish varied over the production years or perhaps the process just wasn't easily repeatable and/or poor quality control of ingredients, temperature, pressure etc.

[ajgriff]

I agree that bakelite quality varies significantly and that some examples are much more difficult than others in terms of achieving a good shine. However many common polishes (Brasso, T-Cut etc) are simply not abrasive enough for the job without resorting to the arduous process suggested by David in post #14.

Cerium oxide in its various grades is an unsung hero when it comes to polishing glass, jewellery, bakelite, aluminium, acrylic and some hard plastics. It is extremely hard and has smooth rounded particles rather than the jagged scratchy grit of silicon carbide. 50g of powder lasts a long time and can be bought on-line for about a fiver. In use a small amount of powder is mixed with water to form a paste. For hard materials like glass power tools are used for polishing but for softer materials polishing is done by hand. It’s not abrasive enough to remove deep scratches (back to the carbide!) but is far more effective when it comes to polishing things like bakelite when compared with the more commonly available polishes.

Alan

[Cathovisor]

Thanks for the mention of cerium oxide: I'd never heard of it but it sounds like a useful thing to have in the arsenal.