View Full Version : I like chemical polishing

05-09-2009, 07:22 PM
I CNC'ed a part for a small project today from polycarbonate and to finish such parts I have been experimenting with chemical polishing. For acrylic and polycarbonate methylene chloride works the best. And boy does it work! It will make even a cruddy tool marked fuzzy part look good. The part turns crystal clear in just a few seconds. All you need is a shallow metal or glass container with a glass cover so you can see what is happening. For this part a cleaned tuna can is perfect. I put a small metal nut in the bottom to rest the part on. Pour in a teaspoon of methylene chloride, place the part on the spacer and cover the can with the sheet of glass. Then direct a hair dryer at the can for a few seconds until the MC begins to visibly evaporate.

In less than 60 seconds the part will suddenly become entirely transparent. Remove the lid immediately and wave some fresh air over the part. After a few seconds carefully pick it out of the container using some needle nose pliers or whatever will work best. Let it dry out for a couple of minutes and that is it.

Naturally, metheylene chloride is on the chemical bad actor list. It can be found at your local supplier of plastics or a shop that makes plastic products. It is no worse than the fumes from gasoline or other volatile solvents but because it has very little odor it is very easy to have enough in the air to be dangerous to health and also pose an explosion hazard. Do this out side or in the garage with all the doors open and you won't have a problem. Do not eat or drink etc... Store outside the house in a metal container.

It can also be used to wipe down the edge of acrylic or polycarbonate after machining and will turn it glass clear instantly. once the part is treated it should be allowed to dry for at least 10 or fifteen minutes. Parts that have been treated and will subsequently be fastened together should be allowed to dry for at least an hour to prevent gluing, unless you want them glued. It is the recommended solvent for gluing these plastics.

The part, which is intended as the rotor in a small centrifugal water pump.



Liger Zero
05-09-2009, 07:25 PM
Ain't plastic nifty?

You can also flame-polish and flame-debur some materials. It's a knack. KEEP THE TORCH MOVING!

05-09-2009, 07:28 PM
Flame polishing doesn't work on polycarbonate unless you first bake it in the oven for about 4 hours to remove the water adsorbed in the surface. If you don't, it forms lots of ugly little bubbles just as it begins to clear.

Doc Nickel
05-09-2009, 08:17 PM
A magnetic water pump impeller, if I'm not mistaken. :D


05-09-2009, 08:46 PM
Yep, magnetic drive. It's nice not having moving seals to deal with.

Bill in Ky
05-09-2009, 09:29 PM
Always nice to read your post's Evan, thanks!

05-09-2009, 09:40 PM
Enjoyed that post Evan, thanks.

I didn't know about that trick. Although, I have flame polished polycarb without the bubbles. Maybe just luck because I have also flamed it and found the bubbles appear. Now I know that I have a better alternative!


05-10-2009, 12:03 AM
They do the insides of some very expensive stainless instrument tubing the same way.. HOW??

I got a four foot piece made into a blow-dart gun. Found a mouthpiece that just fit the tubing. It's shiny too..

Seems it was over a grand per hundred feet. I ordered special roller benders to put it together. I forget why it was spec'ed. I had a piece of titanium stuck back behind the door to make me a large bore toy.. had to turn it in, had to account for every scrap of it.

05-10-2009, 12:19 AM
They do the insides of some very expensive stainless instrument tubing the same way.. HOW??

Stainless steel is electropolished -- very different process.

05-10-2009, 12:41 AM
That is something else I intend to try. The thing that has held me back is dealing with boiling hot acids or alkali liquids. I'll have to build a "hot glove box" to do it in .

05-10-2009, 12:50 AM

Question for you. I have dealt with chemicals some to little in the past, and chemical polishing has always baffled me. In reality is the chemical etching the plastic, as in removing all parts of the plastic the same amount (or close) or does it actually etch the plastic down to a flat surface ? I think in my little brain the what is going on is the machine marks are being etched away leaving the super clear nice finish, however the texture, or finish will still be the same or very close with maybe more of round hills and valleys verses sharp tool marks.

So the chemical is etching the material down so that the tool marks left that are causing the light to be scattered I guess you could say in the clear material is removed enough so that it once again passes though giving the crystal clear look... and it does look great.

Magnetic drive motors are pricey, I have a little giant that I paid to much for used, and retail ( lowest price I could find) new was over $800, used to circulate the solution in my anodizing tank...when I was doing that.

05-10-2009, 12:51 AM
That is something else I intend to try. The thing that has held me back is dealing with boiling hot acids or alkali liquids.

Evan, I saw an electropolishing recipe on rec.arts.metalworking, and I seem to remember that it was a dilute sulfuric acid or phosphoric acid electrolyte.

I'll have to see if I can find it.

05-10-2009, 01:38 AM

Methylene chloride is extremely volatile. It evaporates as fast as ether and is about as dangerous which is why you don't want to do it inside. That said, gasoline also will produce an explosive mixture in air down to about -46 degrees.

When you give the container a short puff of hot air from a hair dryer it accelerates the evaporation of the MC. It forms a saturated vapour in the container and condenses on the part. That forms a thin layer of very active solvent on all the surfaces of the part, even inside small holes. The solvent simply dissolves a very thin layer of plastic which flows out like a coat of paint. When you remove the lid this very thin layer evaporates immediately leaving behind a leveled and clear surface. It doesn't seem to make any noticable difference to dimensions but if you leave the part in the vapor it becomes very soft very quickly. What I haven't tried yet is to dissolve some polycarbonate in the MC and try painting a surface with it.

05-10-2009, 05:24 AM
Solvent polishing used to save hours of pratting about with wet and dry papers in successive fineness of grades and polishes after machining prototype TV remote control lenses. It did of course enable us to get the designs wronger faster.

Regards Ian.

Your Old Dog
05-10-2009, 07:42 AM
I guess it goes without saying that this should effectively eliminate nearly "any" stress risors in the project caused by machining. Pretty neat process.

05-10-2009, 07:53 AM
Thanks Evan

That makes more sense now, and very simple. I don't know why I didn't think of the solvent dissolving the material and keeping it in the solvent kind of like a carrier, I had etch "etched" into my brain I reckon.

05-10-2009, 10:35 AM
If you don't have access is to a plastics supplier, McMaster carries WELD-ON 3 (http://www.mcmaster.com/ctlg/DisplCtlgPage.aspx?ReqTyp=CATALOG&CtlgPgNbr=3361&RelatedCtlgPgs=3362,3361&CtlgEdition=115&ScreenWidth=1280&McMMainWidth=873&sesnextrep=383362669940953) which is methylene chloride. (I'm not 100% thats it is the only ingredient without the can in front of me). We use it an acrylic cement. Very handy in that you can clamp up your work, apply it with a syringe, and it will wick under the pieces.

Evan, thanks for the tip on the polishing with it.

05-10-2009, 12:00 PM
I get to contribute!!!! Yay!!!!

Take a look at:

Back in 2003 I was helping a good friend build a brewing system for beer. I basically helped him build the control box and do the wiring. We got to talking about electropolishing and I did the requisite research to find the formula for stainless.
It is ridiculously easy. I started with lab grade chemicals in order to eliminate my own stupidity. Nowadays, I'd use off-the shelf chemicals since it is such a simple deal.

05-10-2009, 05:51 PM
At work I have an old electro polishing rig that came out of Tektronix. Pretty small, only good for small parts. Might hold a liter beaker.

05-11-2009, 07:43 AM
Evan, How did you keep the plastic part from sticking to the bottom of the tuna can?

05-11-2009, 08:52 AM
I set it on a small metal nut, with the part balanced on it. I found that using a knife edge support isn't a good idea as it will sink into the plastic slightly. I am still working on a more reliable and universal support, perhaps a bed of pins? It would be an excellent application for a small scale antigravity levitator but I still can't get mine to work reliably.

05-11-2009, 02:50 PM
a small scale antigravity levitator

Would loan you mine but the anti-matter leaked out of the containment field and liked to made me wet myself when it popped.

05-13-2009, 11:24 AM
i am building a custom tailight for a motorcycle project and i have an old lense out of a tailight that was replaced on my wifes horse trailer.would it be possible to use your method to remove the surface granulation/fogginess from the old lense?
thanks for the post,

leon holmes

05-13-2009, 12:24 PM
There is nothing I like better than an experiment so I tried the process on an original 1959 tail light lens from my Land Rover.

Yes indeed, it works. The lens didn't seem to want to clear up though from just sitting in the vapour so I just poured some MC over it and Voila! Just like new again. If you pour the liquid over the plastic you should let it dry for about ten minutes before touching it. Just leave it in the container with the lid off and let the MC evaporate. This is one method that should only be done outside because of the amount of MC vapour it will produce.

This looks like a real winner for restoring old plastic lenses for classic vehicles.

Your mileage may vary but as long as you don't overdo the treatment it shouldn't destroy the part. All you want is to evenly wet the surface of the part.


If the features look a bit soft it is because I left it in the vapour quite a while, perhaps 3 or 4 minutes waiting for it to clear. Next time I will leave it in about 30 seconds out side up and at the end of that time then douse it on the outside face and ventilate.

It still looks good on reflectance and especially on transmission.


Gee, it just occured to me that my Land Rover is now half a century old. I've only owned it for 34 years though.

Michael Edwards
05-13-2009, 12:39 PM
Hmmm, I might have to try this on an old pair of hazy eye glasses.


05-13-2009, 02:11 PM
I already tried it. It doesn't work because of the hard plastic scratch resistant coating. However, it works beautifully on lexan visors in face shields. I made sure that it didn't compromise strength by testing one to destruction afterward and I couldn't tell any difference to one that hadn't been treated. This is a good thing because I have two of the Bionic masks and the visor replacement is expensive.

Michael Edwards
05-13-2009, 02:42 PM
That's too bad, woulda been handy. Thanks for the info.


05-13-2009, 03:03 PM
thanks for conducting the experiment.seems to be really effective,now to see if i can find some mc locally.
might be cheaper for me to just buy a new trailer light and cut it up.
thanks for postin,

leon holmes

john hobdeclipe
05-13-2009, 04:32 PM
The headlight lenses on my Toyota Tacoma have become hazed and foggy, with a network of tiny cracks on the inside. Toyota wants $120.00+ each, which really bugs me...the truck isn't even ten years old yet!

So I'll give this a try. What've I got to lose.

05-13-2009, 05:30 PM
It may not work. The headlight covers are without doubt polycarbonate but they may have a hard coat finish on top of that. If so it will not work, same as with the eyeglasses. Instead, it will make matters worse. If you were able to remove the hard coating then you could apply the MC and it would probably work well but that would require some careful wet sanding to avoid changing the shape too much. The headlight cover acts as a part of the lens system.

The inside may or may not be hard coated, it's worth trying a little bit on an edge. I recommend a couple of practise runs on something that doesn't matter before tackling an expensive to replace part.

05-14-2009, 12:36 AM

Who know it may work?

05-14-2009, 12:45 AM
I am sure it will work, it's just a polishing compound. The problem with using any sort of power polishing equipment is that it is exceptionally easy to over heat and burn the plastic. If the buffing pad is allowed to run dry for even a second or two the heat developed is amazing. That's because it has nowhere to go. Plastic is a good insulator and thermal conductivity is directly related to electrical conductivity.

05-14-2009, 01:03 AM
I used the red Mothers sponge from walmart, and the headlight polishing compound to spruce mine up, just took a few minutes, and the way the sponge is I can't see how you could overheat it. I have polished plastic on a buffing wheel, it has a huge learning curve, soft fluffed wheel raked often, easy on the pressure, and a blue compound if I remember correctly from caswell, they have outstanding selection of buffing compounds, wheels etc, and also a great writeup on how to.

The buffing didn't remove the small pits from gravel, grit etc, however it removed the haze, and greatly improve the look, and most of all the amount of light at night.

Edit to add, I have read all over you need to add a UV protection wax, polish etc or the haze will come back fast ( from the sun) I need to do that soon, I guess when you polish the plastic cover it removes the factory protection. This is on a 2002 Honda Accord


john hobdeclipe
05-14-2009, 10:03 AM
The polishing compound will work on the outside of the lenses, and I intend to do that when I take them off. But the crazing I see seems to be on the inside, where all the prismatic ripples and stuff are moulded in, so I guess I'll try solvent polishing the insides.

As I stated, I've nothing to lose, as the alternative is to spend nearly $250.00, and it has to be fixed soon anyway.

05-18-2009, 07:32 PM
Does anyone know if trichloromethane will work for this? I have about 4oz that I got years ago because I was told it was a great solvent for welding acrylics. I have never used it. A teaspoon at a time will go a long ways, and I would love to have the ability to easily polish my acrylic and polycarbonate engravings.


05-18-2009, 07:40 PM
There is one way to find out.

05-18-2009, 08:37 PM
There is one way to find out.

:D Well that is the easy answer! I was hoping someone would have first hand knowledge, but if no one pipes up, on my next days off work I will give it a shot on some scrap and find out..

I assume that the trichloromethane is much more difficult to find then dichloromethane, as far as I can tell though, I have no plastic distributors close by enough to get a hold of DCM.

I looked up the MSDS for weldon-3, and it is listed as 85-99% DCM, 1-15% something I can't recall, I am not sure if it will truly work for this... If the TCM doesn't work, I will order some from mcmaster with the next order and see.


11-26-2009, 06:39 PM
They do the insides of some very expensive stainless instrument tubing the same way.. HOW??

I got a four foot piece made into a blow-dart gun. Found a mouthpiece that just fit the tubing. It's shiny too..

Seems it was over a grand per hundred feet. I ordered special roller benders to put it together. I forget why it was spec'ed. I had a piece of titanium stuck back behind the door to make me a large bore toy.. had to turn it in, had to account for every scrap of it.


Probably they thought you were fixin" to make 'mountain dew'.

11-26-2009, 08:59 PM
Neat job Evan,the last place I found MC readily availible was aircraft paint stripper in spray cans.

11-26-2009, 09:35 PM
I've had methylene chloride in a glass container for years, and it didn't leak away. Then another glass container with some 'disappeared' within a few months.

Both lids were on tight, but one must have had a seal disc in it that was permeable to MC. In future, I'll have to make sure the container is appropo for the contents, which mostly relates to the lid and seal. I've had the same problems with acetone.

I have been meaning to build myself a hooded work area specifically for working with solvents, spray paints, soldering, silver soldering, etc.

Not meaning to hijack this thread, but dealing with fumes is related-

Imagine a cardboard box with the top gone. Around the opening is ducting which blows air across the surface of that opening from all sides. Now put that box on a desktop with the opening facing you. At what is now the back of the box (used to be the bottom) you put a row of fans across the bottom and top, plus one row right up the middle. These are the exhaust fans.

Fresh air is brought in and blown pretty much across your face from both sides and the top and bottom. If your face is pretty much in the middle you shouldn't feel much draft because the air streams pretty much cancel. The rear fans draw that air into the box and pull it out, along with fumes. Any vortexes created within the box should be confined to the sides, where the volume is the largest it can be (assuming two basic vortexes) and that keeps windspeed as low as it can be. Both heavier than air and lighter than air products are prevented from dissipating out the front of the box.

Outside, the vent is positioned to prevent it's output from going back into the inlet side. Not sure how I'm going to do that, but I'll have to study the airflow in the area where my vents have to be.

Of course, the final structure isn't going to be a cardboard box, but essentially that's what it would be. The bottom part is the worktable, the top and sides make up the hood, and whether or not I need to make a fold-out top to extend the top of the hood- it might be a good idea to do that, since it could become a front door basically to seal off the whole thing. Something inside could be left to air dry for example, without having to have the fans on all the time. I'd run the fans, spray paint (if that's what the job is at the moment) then close the front off. At that point I could kill the fans, or have them going more slowly- at any rate I could run the fans again for a minute or two before opening the door. With the door closed, I can maintain a set temperature within the volume that is independent of room temperature, which will often be much cooler than needed for paint, epoxies, etc.

I'm at the point where I can't afford to be breathing much more soldering resins, spray paints, solvent fumes, epoxy fumes, flux vapors- whatever. Very little of what I do would require a space larger than a typical desktop, so that gives a scale to what I want to do.

This is my conception and it surely can stand some criticism. Feel free. For various reasons, I need to be able to do stuff like this within the house, in the basement shop, so the implementation of this idea has to work quite well. I can envision a heat treat oven being a part of it, where the fumes from that can be extracted through the same system.

11-26-2009, 09:58 PM
I am at the point where I might just build a glove box so I can handle about anything. The other thing I would really like to have is some motion scale reducing waldos for small work.

11-27-2009, 06:18 AM
Awsome thread. I recall I once tryed chemical polishing as a spur of the moment thing using my fuzzy mind as a refrence. For some reason acetone came to mind so I tryed wipeing some acrylic down with dykem layout remover (Its mainly acetone+a few other solvents according to the bottle)

Nodda, nothing, zip zilch. Did'nt seem to even phase the acrylic (Mind you I was using a damp rag and just wiping it down)
I'll have to try with the proper chemicals someday :) I did pick up a small glass bottle of that metheylene chloride a few months ago, i'll have to try that.

BTW evan: 'thermal conductivity is directly related to electrical conductivity.'

While this is a great rule of thumb, there are a few materials that violate it.
Diamonds iirc are actualy a good conductor of heat but also a super insulator of electricity.

11-27-2009, 06:31 AM
I was referring to metals only. Diamonds are indeed an exception as they conduct heat as much as 5 times better than silver. It is likely the reason that they are referred to as "ice". Apparently picking up and holding a very large diamond is like holding an ice cube. Of course, I don't know that from personal experience. It is also why diamond makes an excellent IR window. It is also a most attractive characteristic for use as a substrate for semiconductors and heat sinks in general.

The thermal conductivity of metals and the electrical conductivity are related not just as a rule of thumb but because they are manifestions of the same physical property, the mobility of electrons.

11-27-2009, 06:50 AM
Ah metals only. that makes sense.
hehe diamond heatsinks, Can't wait.
Actualy what I want is the 300c+ temp that diamond semiconductors can reportedly run at. I'll make a little steam engine for my cpu to run the pc fans.
Or maybe turn my graphics card into a coffee maker? the current one I use is rated at like 400W full draw... (Graphics card, Not coffee maker)

PS: I LOVE that impeller. Stuff like that just really make me wanna CNC my mill asap, Of course that means a giant project, lots of hard work, and learning yet another software package and type of design, and im still learning just the basics of the 3 billion tricks and techniques that make what seemed hard/impossible yesterday easy today.

Iv actualy kinda wondered about making a cheesy optical encoder+stepper+avr setup that could bolt to my rotary table and X axis, and just take the rotary table handwheel movement and move the X axis an exact ratio of the rotery table, Much like the fancy 'hexil' gearboxes some rotary tables have, but without the requirement of well, gears and physical connection :)

but I would likey be better off just going full CNC.

11-27-2009, 07:06 AM
On the subject of chemical polishing with MC I found one thing that makes a big difference to how well it works. The part needs to be colder than the MC. If the part is at all warm, say from working in warm temperatures in the shop as I was this summer, the MC won't condense on it very well. It will work best if the part has been refrigerated for a little while but not to the point that it condenses moisture itself.

11-27-2009, 10:19 AM
I'm curious about the electropolishing. The included links didn't say much in the way of the power needed. It looks like he is using a power supply out of a computer, which would imply pretty low voltage, 3, 5 or 12 I'm guessing. Does anyone have the real numbers needed?

I've got some little stainless parts that would look great if they had a nice shine to them, but they are too intricate to hand polish.


11-27-2009, 12:02 PM
This mixture works well. I would try a couple of car batteries in series. Carbon electrodes are available at a welding supply. Strip the copper with straight muriatic acid.

65% Phosporic Acid (85% solution)
25% Sulphuric Acid (18N)
10% Isopropyl Alcohol (90%).

Also these, from patent # 3970529



To make a liter of solution the following formulation was prepared:

600 ml concentrated phosphoric acid
400 ml polyethylene oxide (MW 400)
0.1 % polypropylene oxide 400-oleate

Electropolish at 170.degree.F, and 15 volts using carbon cathodes. The product was bright after 3-5 minutes of treatment. The above addition agents formed a foamy film over the aluminum part, so that it was not attacked while being polished.


To make a liter of solution a composition as follows was utilized:

600 ml concentrated phosphoric acid
400 ml polypropylene oxide (MW 300)
0.3 % polyethylene oxide 400 -- stearate

Electropolish at 185.degree.F, and 25 volts. The product was polished for five minutes using insoluble carbon cathodes, producing a bright and smooth surface. The additives protect the aluminum alloy part processed from chemical attack while being polished.


To make a liter of solution there was combined:

500 ml concentrated phosphoric acid
500 ml methoxy polyethylene oxide (MW 500)

Electropolish at 180.degree.F, at 30 volts, using carbon cathodes. The product was bright and more polished after five to ten minutes of treatment and a bright film was apparent uniformly over the part.

12-25-2009, 08:18 PM

In the spirit of experimentation, I tried MEK. I squirted a little bit over a small piece of Lexan, left it for ~30 seconds, then tipped it to pour the remainder off. Bummer! It left the surface a milky white color. Too bad because it is a great plastic "glue" and readily available by the quart from Home Depot.


12-25-2009, 09:23 PM
Lexan has poor resistance to a wide range of chemicals. Even plain isopropanol will seriously affect it causing the surface to craze and greatly weakening it's impact resistance. The only alcohol that is safe on all plastics is ethanol, the drinking kind.

Roy Andrews
12-25-2009, 10:17 PM
Evan, you forgot the most important characteristic of a diamond. the ability to facilitate in the manipulation of the fairer sex.

12-25-2009, 10:31 PM
Liquor is quicker....:D