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I used sandpaper of different grits to sand the surface of plastic. The surface is smooth but it looks dull after sanding. I read that one could use toothpaste to remove the foggy look but it did not work. I also tried vinegar with baking soda. When the plastic was inside the vinegar+baking soda bath, it looked transparent and shinny. However, when I took it out to wash it with water to clean the smell, it looked shinny and transparent when being washed but it became foggy and cloudy again after washing. I put it back to vinegar+baking soda bath. It became transparent again but when I used a clean microfiber cloth to clean it, it became foggy again. Looks like some kind of thin film on it. Putting it in vinegar+backing soda bath for about 8 hours did not help. What caused the foggy/cloudy look? Would you please tell how to I make the plastic fully transparent?

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    $\begingroup$ It is still porous and therfore diffuses the light. When wet, the solution fills gaps and roughness. With somewhat matching refraction indexes, the transparent looking is recovered. I am afraid it will stay opaque. Perhaps grinding with special creams and powders of very fine size might help. But those are expensive niche products. Likely new foils are much cheaper. $\endgroup$ – Alchimista Jan 23 at 15:48
  • $\begingroup$ Thanks. Do you have any good recommendation for special creams and powders? I tried "Flitz Green Metal, Plastic and Fiberglass Polish" but it made things even cloudier and dull looking. What sandpaper grade do you recommend to use to start sanding? I started from 120 grits. I wonder if this created more porous and made it more difficult to achieve good transparency. $\endgroup$ – sunflower Jan 23 at 16:03
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    $\begingroup$ Which plastic? There are many and their properties are very different. As are the reasons why they appear cloudy. $\endgroup$ – matt_black Jan 23 at 20:30
  • $\begingroup$ See also Matt Black comment below. Also, it is possible that depending on the plastic mechanical properties you can't recover a smooth surface. In the sense that no matter how fine you go, the coming out particle are always such. To give you an exaggerate picture: you can polish quartz but not a polystyrene foam block. $\endgroup$ – Alchimista Jan 24 at 9:52
  • $\begingroup$ Sorry, I have no idea what kind of plastic is used to make those boxes bought in store. I am trying to clean up scratched surfaces of transparent boxes to make them look brand new again. I read that one could use sandpapers to remove the scratches but they are still there. Does that mean I need to use some kind of compound or wax to fill in the scratches while sanding to make the scratches disappear. What kind of compound or wax do you recommend? $\endgroup$ – sunflower Jan 26 at 15:29
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The tiny scratches on the plastic surface reflect and diffract light in different directions, so edges of objects behind the plastic become distorted, and the clear areas of the plastic are less clear.

There are several ways to make the plastic more transparent.

  1. Use a finer grit sandpaper - really really fine: 1500 grit or finer, if you can get it. Plastic abrasive sponges are more gentle and can be used wet, so may create smaller, less deep scratches.

  2. Use toothpaste to "sand" the plastic in the first place. You have to get the paste to be pasty thick, not watery thin. It will reduce scratches, but if you don't put deep scratches on in the first place, it will be easier to shine up a plastic surface. Toothpaste will reduce the shininess of a perfect surface, but usually will improve an ordinary surface. There are commercial pastes for cleaning and shining stovetops that are more abrasive, and you can make your own with sodium bicarbonate (no vinegar) - but you have to give it some energy, some elbow grease, and it takes a while. You could use a buffer wheel on a power drill.

  3. Filling in the tiny scratches with a liquid helps even out the surface, but water evaporates and the effect does not last. An oil (e.g., silicone) or petroleum jelly (very thin layer) might last long enough for you - days, maybe weeks.

  4. Depending on the plastic film, you might be able to heat it slightly to allow the scratches to deform slightly and become slightly wavy instead of sharp valleys and projections. This will require a lot of experimentation to get enough heating without distorting the plastic beyond your requirements. Hot air or hot smooth metal, or perhaps hot water or steam might work.

  5. The least likely method to work is using a solvent to smooth out the surface. You might get some immediate effect that looks like an improvement, but any solvent that dissolves the very top surface will also diffuse into the plastic and when it evaporates, you will have a plastic that is disturbed even deeper, and will be foggier. Working this approach further suggests that you could use what would be better described as a plasticizer, a high molecular weight liquid that is compatible with the plastic, but does not migrate (out, usually, but in here), buff the surface and quickly clean the plastic to remove all the solvent/plasticizer. Now, since you probably don't have access to different kinds of plasticizers, you could try some vegetable oil plus a little of a more powerful solvent, like a few percent of alcohol, or mineral spirits, or even acetone, and you could add some sodium bicarbonate for a little abrasion, and use a buffer wheel.

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  • $\begingroup$ Thanks James. 1. I sanded from 120 to 5000 grits. Is it better to start from 1500 grit? 2. Do you mean sand with paste first and then 1500 grit and up sandpapers? Any recommendation on which paste to use? I used Sensodyne Pro-Email. 3. When washing, at what temperature should the water be? I wonder if too cold or too hot would affect the transparency. $\endgroup$ – sunflower Jan 23 at 16:17
  • $\begingroup$ The problem is that the plastic might be cloudy because it is porous not just because the surface is rough. This might explain why it isn't cloudy when immersed in some liquids as they "fill in" the holes with something other than air, reducing the scattering. $\endgroup$ – matt_black Jan 23 at 20:31
  • $\begingroup$ @matt_black: Good thought. If the plastic is cloudy to begin with, polishing might be useless. sunflower is asking specific questions, but providing no information to work with! What kind of plastic are you working with, what dimensions, for what purpose, what have you tried, and WHY? $\endgroup$ – James Gaidis Jan 24 at 14:50

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