Polishing old cloudy transparent plastics

[YoungManGW]

Toothpaste works a treat.
Regards,
Richard

[ajgriff]

Originally Posted by Terry_VK5TM

Many years ago, we used to use toothpaste on Arcade machine perspex.

May not work now with the change in toothpaste formulations

Modern toothpastes still contain mild abrasives and can be used for final polishing of plastic. Eucryl Smokers Toothpowder is slightly more aggressive and is still available cheaply in the UK.

Alan

[Skywave]

This is the same issue as with Bush TR82s, where the large transparent knob, (sometimes incorrectly described as the 'dial') goes milky right through and no amount of polishing makes an iota of difference

Originally Posted by Karen O

I wish I'd known that seven years ago! I spent a whole weekend polishing away at my TR82C knob with nothing to show

Ditto here! I have TR82 in same condition; been here for ages. I've often thought about how to fabricate a replacement transparent knob, but doing that will be v. difficult - and for me, simply not worth the huge effort trying to achieve a satisfactory result - if one is achievable at all!

Al.

[Junk Box Nick]

Originally Posted by yesnaby

Edd from Wheeler Dealers did this successfully to car headlights, though the process started by sanding them to become very rough and matt, then several stages with finer and finer sanding/polishing. Seemed to work well.

I have used this process on headlight lenses with success using very fine grades of wet and dry paper.

[ajgriff]

The principle abrasive used in proprietary plastic polishes is cerium oxide. This can be acquired economically (minimal packaging and marketing hype) in the form of a white powder. I bought 50g for £5 about five years ago which may sound a lot for a couple of teaspoons full but it really does go a long way. From memory I've used it to remove scratches from and polish turntable covers, meter lenses, CDs, bakelite 'phones, plastic 'phones, aluminium trim, sheet steel, car light lenses, mirrors & a car windscreen. All this and I still have about 10g left. Incidentally, power tools are a practical necessity for glass but plastics and soft metals must be done manually.

Tractionist also mentioned using wet & dry paper for deeper scratches ie, ones you can feel with a thumbnail, which is realy the only way to approach the problem. It does take a leap of faith to see a transparent surface turning opaque as a deep scratch is erased. However, it's very satisfying when bright and shiny transparency is restored with polishing. The abrasive used for wet & dry paper is usualy silicon carbide which is an exremely hard compound just like cerium oxide. The difference between them is that the carbide particles have a sharp jagged surface whereas the oxide particles are smooth and rounded.

Alan

[mole42uk]

I second the use of Brasso, used it on several of my Heathkit meters with excellent results.

[MrBungle]

Have just tried brasso ironically on a junk Heathkit GDO knob (the big plastic bit). My word that works pretty well so far!

Am using a cheap Tesco microfibre cloth with it.

[ekjdm14]

I've used Brasso with great effect years ago on a Perspex ve*icle window that was completely yellow/opaque and "gritty" feeling. Have used it to good effect on other automotive related things too & would suggest it's likely to be good for this hobby also. With the caveat of trying on a small area first to make sure not to kill a rare part, and protecting with plastic polish afterwards.

[mole42uk]

After the Brasso treatment, I use some Pledge furniture polish, sprayed lightly onto a cotton duster, to wipe over the surface. It keeps the gleam for ages!

[Glowing Bits!]

Brasso is good for removing most of the yellowing.
For better, long lasting results, wash the plastic to remove Brasso traces, then spray on a bit of Plexus followed by a quick buff, the finish will stay shiny for a good deal of time.
I'm going to see how long it takes someone to ask what the magic spray is, it's a secret!
Rick.

[Glowing Bits!]

Here is an MB60 dial (the TR82 but earlier) after the Brasso treatment, it was yellow to the point of being useless.

[ITAM805]

Edd from Wheeler Dealers did this successfully to car headlights, though the process started by sanding them to become very rough and matt, then several stages with finer and finer sanding/polishing

I did my badly yellowed Astra headlights the same way and finished off with mag wheel polish applied to a damp buffing mop, worked a treat

[Bazz4CQJ]

I've just spent a few minutes on Google re "polishing plastics"; there are numerous threads, many to do with car headlights, caravan plastic windows etc. There seems to be little consensus. Some think that T-cut is wonderful, others say that it (and Brasso) contain too much ammonia which is bad. Some say cerium oxide is good on plastic, others that it should only be used on glass. I guess that one issue here is that there are quite a lot of plastics and we seldom know for sure which one we are dealing with.

B

[Uncle Bulgaria]

I'll reiterate my previous post that Brasso/Silvo causes crazing - I just tried it on some acrylic in case my previous experience wasn't because of the polish but some other effect. Even during the polishing, micro-cracks appeared on the edges, so do test before trying it out! No doubt the particular type of plastic being polished matters too.

I'm going to try getting hold of some cerium oxide, which sounds the bee's knees. ajgriff - did you mix it with water?

[ajgriff]

Originally Posted by Bazz4CQJ

Some say cerium oxide is good on plastic, others that it should only be used on glass.

B

Just like wet & dry paper cerium oxide comes in different grades from coarse to fine. Vendors of the coarser auto-glass grades warn that their product may leave fine scratches on softer materials like plastics. In theory for badly scratched plastic you would work through the grades finishing with very fine for an 'optical' finish. In practice this is completely unnecessary for most purposes. My 50g packet (see earlier post) is a medium coarse glass grade and produces an excellent finish on plastics without further polishing. This can be improved slightly by finishing off with T-Cut, Brasso or even toothpaste depending on how far you wish to go. Happy to take some photos of my efforts if there is sufficient interest.

Alan

PS The perceived wisdom with cerium oxide is that you make a paste with water. However, I use a damp cloth and increase the wetness as I go which seems to work well. Just leave any unused powder to dry out for future use.

[Argus25]

The real answer to this question depends on what kind of finish you are satisfied with.

Brasso for example gives a about as coarse a result as the coarse grade of the Novus polish I cited. That is nowhere near good enough for a mirror quality finish. This is why if you then add another masking compound like furniture polish to the surface it looks more glossy and masks the fine scratches from the abrasive in the Brasso .

If adding any kind of surface oil improves the appearance, it is just an indication that the polishing job is inadequate and the surface still is too rough. So like I say, if you want the best possible job, get the Novus, if you are happy with an inferior result, go ahead and use the Brasso.

[ajgriff]

I would guess that Novus contains a very fine optical grade of cerium oxide.

Alan

[Graham G3ZVT]

I polished the lens of a Sony Xperia smartphone with Silvo wadding. I had to rub it very aggressively and didn't think it would end too well.

By lens I mean the exposed part that is really a filter for protection rather than the actual lens. The phone was a replacement from my company and the previous owner did not look after it, and allowed the lens to rub against a hard surface.

The images all looked as if they were taken through a gauze. Close inspection suggested that the damage was to the photochromic coating, so my intent was to remove it completely.

It worked beyond expectations and the camera was as good as the original one. The loss of the coating had no detrimental effect.

[MrBungle]

Well time to report an outcome. I picked the GDO I am in the middle of re-kitting and used Brasso and a microfibre cloth then used toothpaste (standard Aquafresh) to finish it off. About 20 minutes of polishing and I got this which I'm extremely happy with.

Before:



After:



I made a new scale by scanning the old one at 1200dpi, set hue to mostly yellow, then increase saturation, convert to two bits (B&W) and laboriously cleaning up the image in gimp on Linux. Printed on the back of a Chartwell log/log paper pad sheet with an inkjet.

[mole42uk]

An excellent result, it looks like a new one!