Removal of 'Tropicalisation' Coating From Chassis = Perhaps Not?

[Anthony]

I do not mean this to sound like a Crossword Clue but here goes. I have recently acquired a WWII BC-191F Transmitter. The Chassis is coated in the 'usual' yellowish 'Tropicalisation Varnish'. It looks like someone has had a go removing some of it. What remains is unsightly with a curious unpleasing patterning - perhaps some one 'had a go' on removing it previously and gave up . Of course this coating, or something that looks much like it, was very widely used in this sort of equipment and it have never previously occurred to me to remove it. It is part of the patina of the units. I also think it contributes to the typical smell of much WWII gear.

I have two questions:-

1. Is it even safe to remove the coating, provided basic precautions against oral ingestion/contact are made. Highly toxic mercurials and organic copper compounds appear to have been used- see http://www.tuberadio.com/robinson/Manuals/ Then look down the list till you come to this file AWA Technical Review - tropicalisation (3.3 Mb ) - many thanks to this site for that fascinating historical reference.

2. How would one go about removing it? I suspect this is the same as asking how you remove varnish from wood or metal plus the toxicities mentioned above. I also suspect that either Dichrolomethane (old now banned paint-stripper) or Strong Alkali might do the job. I would not like either dripping on the unit (!) and would steer well clear of both unless the chassis was stripped of all components in the first place.

(FWIW, acetone, isopropanol, WD-40 or Carburettor Cleaner do not touch it).

Has anyone encountered this problem before? My bias is to leave well alone or to strip off all components before using these highly corrosive treatments. But maybe someone with experience of this can advise? Thank you.

Anthony M0WWV

[Bazz4CQJ]

Regarding toxicity, I would guess that you are right in identifying contact/ingestion are the main concerns. Inhalation seems less likely as any volatile material would have been less suitable for original use, and even if it had been used, would have gone by now(?).

Regarding solvents, acetone is quite a powerful solvent, so if that is not working then that is discouraging. One of the 'green' paint/varnish strippers which has come on the market in recent times is based on benzyl alcohol (the one I use comes from Wickes). This stuff has the consistency of PVA adhesive, i.e non-drip, and although quite slow to act, I have found it surprisingly good on some coatings (not all). May be worth a try if you do want to proceed.

B

[brenellic2000]

It may simply be shellac, which was often used to 'tropicalise' (ie. protect from humidity) and is easily removed by methylated spirits.

[frankmcvey]

Further to post 3, shellac is still widely available, and it's very easy to reapply as well if you wanted to restore it. You can even get it in aerosol cans nowadays. I've never used it in that form, but it seems to me it would lend itself you your job. Assuming, of course, the original WAS shellac - test it in an inconspicuous area with meths as suggested by Brenellic.

[greenstar]

If IPA doesn't shift it I would doubt meths would - it's a similar solvent.

[dseymo1]

How about cellulose thinners?

[HamishBoxer]

Silly question but why does it need removing?

[Skywave]

Originally Posted by HamishBoxer

Silly question, but why does it need removing?

The OP has said:

Originally Posted by Anthony

It looks like someone has had a go at removing some of it. What remains is unsightly with a curious unpleasing patterning - perhaps someone 'had a go' on removing it previously - and gave up .

All of that sounds like a good reason to me.

Al.

[Anthony]

Many thanks for the very helpful comments. I have made a bit of progress.

Firstly, 'Why does it need removing'? It looks dreadful (see photo attached) also I am a bit jumpy about potential toxicity of the flaking varnish and exposed aluminium. I thought that the 'snake-like' pattern is in the varnish but in fact it seems to be corrosion of the aluminium underneath (see below).

I was hasty in saying that acetone does not remove the varnish. If it is put on a cotton ball and left for about 15 mins it loosens the varnish enough for removal by a 1/4 inch wooden scraper (slow work!). Even more effective is my wife's nail enamel remover - Cutex. This is more viscous and slightly quicker - it too contains acetone. I am trying to do this without these aggressive solvents running inside and ruining components.

So removing the varnish is possible. I have further cleaned the corroded aluminium with isopropanol to remove any residual fungicide. (Double gloved and careful disposal of all waste and tools).

But what is underneath is corroded in the curious snake-like pattern shown in the photo. (The varnish-free area in the photo is where it had flaked off and not from my efforts). Acid as white vinegar has little effect on the corrosion pattern and, reluctantly, only a light abrasive pad (Scotchbrite) restores a reasonable finish.

So I now need to set aside 3-4 hours for methodical varnish removal and light aluminium abrasion. Any suggestions for avoiding an abrasive welcome. Use of strong alkali is out because any drips would wreck components and it could never effectively be washed off.

Thank you for any further thoughts.

[Anthony]

I forgot to say that the components are in good condition and just dusty. There is little or no corrosion or other damage. So the unit is well worth saving. It's just the chassis that lets it down.

[emeritus]

My 1950's edition of the "ITT Reference Book for Radio Engineers" has a section on tropicalisation. It says that bare metals such as brass and aluminium should be treated with lacquer, but gives no details about the type of lacquer to be used.

At Plesseys, one of out projects used cast aluminium alloy Eddystone boxes that had a fairly rough and dull surface finish as delivered. Our workshop polished them to almost a mirror finish using a rag mop wheel on a bench grinder, but I don't know if this involved the use of abrasive.

[HamishBoxer]

Thank you for the picture I certainly see your point in wanting it off.It is indeed a complete mess.

[Anthony]

Both of the main valve compartments have now been virtually cleared of the Tropicalisation coating (see photo). The labels have been left alone. This took over 3 hours and although the result is not perfect, I now feel more confident about work on the electronics. (Precautions were taken against contact/ingestion of the material - thick gloves, eye protection, surface protection, change of clothes and washing.)

I still do not know what fungicide(s) were used. It appears they were first applied in solvent, allowed to dry and then varnished. It also seems that napthenic acids (see Wikipedia), possibly as the copper salts were widely used. And this fits in with the characteristic smell - would others agree that it is similar to old Green Cuprinol? The napthenic acids probably varied a lot in exact composition and not all the 'tropicalisation smells' are, to me, the same. Napthenates would have been widely available at the time. Any other ideas?

I think I shall leave the chassis alone for the moment and start fixing the broken antenna relay!

Thank you to those who have stayed with the Thread.

Anthony M0WWV

[G3VKM_Roger]

Hi All,

Just my thoughts but US WW2 equipment (and post-war) is frequently marked "MFP" and a date. That stands for Moisture and Fungicidal Protection. There's an occasional reference to MFP treatment in the TM series of US manuals so I would guess MFP has a TM or part thereof devoted to the process, maybe someone with more info knows the right publication.

The TX looks a lot better for the cleaning. Hope you got a set of valves with it, as they cost a King's Ransom!

73

Roger/G3VKM

[Anthony]

Roger

That's a great idea. I'll follow up but it may take a bit of time since I am going on my hols.

Best.

Anthony

[ColinTheAmpMan1]

Originally Posted by Anthony

It also seems that napthenic acids (see Wikipedia), possibly as the copper salts were widely used.

Sorry to be a chemistry pedant, but I do have two degrees in the subject. The correct spelling is "naphthenic" with an extra "h" and the "phth" is pronounced "f-th", not "p-th". Pedantry over.
Colin.

[Anthony]

OK Colin you caught me out. But I only have a degree in (impure) Biochemistry. The mis-spellng is not uncommon, which does not make it right. Mea culpa.

Interestingly there is a lot more to this although it is poorly documented. 'Tung Oil' was frequently used as part of the MFP process.

Do you have some definitive info.? The whole subject really deserves some 'research'.

Anthony

[ColinTheAmpMan1]

Regrettably not, Anthony. I am a PhD Organic Chemist and work in Medical Research at Imperial, London. My specialisation some time ago was mass spectrometry. I'm a bit of a gopher these days, but I only work 2 days a month and passed retirement age nearly 4 years ago.
Colin.