Why is my clear resin part opaque after sanding but transparant when wet?

Hi there,

I’m working on a project that requires very transparant parts. I already printed the parts on my Form 2 using Clear Resin. I washed them in isopropyl alchohol and post cured them. Afterwards I started wet sanding very thoroughly from 320 to 1500 and plan on sanding to 2500.

The part is becoming much more transparent when wet but when it’s dry it is opaque and even less transparant than a part that hasn’t been sanded at all. Why is that? And how to a get the same “wet” transparency when dry?

I planned on finishing the sanded part with a thin layer of very clear epoxy. But I’m not certain if that will fix it or if it will just keep the part opaque.

Any thoughts?


Well that’s… physics.

You sand something and add water that act/behave like a varnish thus eliminating all sanding mark…

Just clear coat it with any glossy varnish and it will be transparent as wet.

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sanding leaves tiny little scratches that refract and diffuse light.

If you keep on sanding, moving from 100 grit paper- to 200 grit- to 320 grit- to 400- to 600, and to 1000 or 1200 grit, you will be making the scratches finer and finer, until you can move to a polishing rouge- which will make the scratches finer than the wavelengths of visible light- at which point, the surface will become optically clear.
That’s how they make Lenses.

OR- Sand down to about 400 or 600 grit, and then apply a thin coat of gloss ACRYLIC.
Acrylic paints will bond better to the acrylic resin than will epoxies.


Thanks for the clarification.

So does it make any difference if you sand till 1500 and then apply an acrylic coat instead of just sanding till ~600?

Not really- if you are gonna clear coat it.

But you do not need to use a clear coat if you sand up to 1500 and then do a polishing rouge.
It will be Actually optically clear in itself.

A gloss clear coat is just filling in the tiny scratches with a clear paint that has high surface tension so its air exposed surface is smooth on a molecular level.

Like when you wet a rock… its just coating the rock with water and the air exposed side of the water is smooth because of surface tension.

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Thanks for your reply. I’m quite new to finishing transparant parts so your answers help a lot.

For my second part, I ideally would sand it up to 1500 and then apply a polishing rouge for the most optically clear result. However, the sanding of my first part took way too long. So I’m looking for a faster way. I tried clear coating but it didn’t give me the transparency I was looking for.

Would it make any difference if I sanded till 600, applied a clear coat, and then polished it for a higher transparency? Or any other way to get a higher transparency than clear coating without sanding till 1500 and polishing it?

Sanding take time… but not usually as much time as folks think.

The trick to sanding is to start course, and work your way up to finer grits incrementally.

Most folks start with too fine a paper and end up spending hours just trying to average the surface out fair.
You use each grit ONLY so long as it takes for ALL the scratches on the surface to be that fine- then step up to the next weight of paper, but you Start with something that will average the surface faster… like 100 or 120.
It only takes 5 or 10 minutes to sand something a form 2 can produce to an average surface with 100 wt paper.
Thereafter, each grit of paper has to remove LESS material to bring the surface to a finer finish.

As to polishing the clear coat- it depends on how rough the part is and how thick the clear coat- in polishing the clear coat you might polish THRU the clear coat in some spots and you will not get a smooth transition between the clear coated area and the exposed resin surface.

Your best bet with clear coat is that if it isn’t optically clear enough with one coat of clear gloss, then you apply Additonal coats of clear gloss.

You are not gonna polish a gloss clear coat any smoother than the surface tension of the clear coat as its dries.

you can also use paste wax- polished and buffed- to get glass smooth clarity.

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Carefully sanding is really the only way. I did that to make a windshield piece for a miniature, took about 3 hours going from low grit up to like 12,000 grit wet sanding.
Also, I used some white oil afterwards to add a bit of polish. I haven’t used any coatings since I wouldn’t want to mess it up and ruin the part, but it would likely be more easily polished with a clear coat if applied correctly.

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