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I have project made up of small triangles of wood forming a polygonal shape. Between each of the pieces of wood is a gap, and I would like to fill this gap with a clear material that can be sanded.

I've tried silicone, but it comes out lumpy and there's no way to smooth it out. Ideally, I think a thick epoxy resin mixed with clear filler might work, but I can't seem to find something like that. I've also seen there is such thing as transparent grout, but finding a small tin of it has been difficult.

How would you go about filling these cracks?


EDIT: Adding details as per request:

  • the overall objective is to put light in the model, and have it shine out the cracks so:
  • clearness matters, as clear as possible
  • strength would be nice, but it doesn't need to be super strong
  • being able to sand it smooth makes a big difference in the final polish and shape
  • it is hollow, but I have already used silicone to back the gaps (just can't fill out to the face and smooth correctly)
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  • You can smooth silicone very well with your bare fingers if you dip them in a bowl of water with dish washing soap. The silicone cannot stick to the soap, so you must keep your fingers quite wet and you should avoid getting the wood wet at the same time. However, you probably won't be able to smooth out every bump or finger print perfectly and even transparent silicone is not see-through clear.
    – Elmy
    Commented Jul 27, 2022 at 5:46
  • That's a neat trick. I'm really trying to get a sandable material though.
    – Seph Reed
    Commented Jul 27, 2022 at 5:48
  • Clear wood grain filler seems like it might be promising: amazon.com/s?k=clear+wood+grain+filler
    – Seph Reed
    Commented Jul 27, 2022 at 5:58
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    How about a mix of wood glue and wood pulp? You can get very smooth and invisible results with it.
    – Joachim
    Commented Jul 27, 2022 at 6:11
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    Can you clarify what properties you want the filler to have? Is a milky semi-transparency enough or do you want something glass-like? Do you have any requirements concerning the strength of the material? How big are gaps (rough estimate)? Is the object hollow or is there a filling that could keep the transparent filler in place while it dries? You also mentioned that you're looking for a sandable material in the comments. Please edit your question and add a list of properties you're looking for.
    – Elmy
    Commented Jul 27, 2022 at 7:00

2 Answers 2

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Being a 3D shape adds some complication. If it was a flat surface, you could dam the outside edges and use any number of clear, hardening materials that flow, like epoxy resin. You could still do that if you orient the piece so one seam is up and level, dam the ends of that seam, and fill that seam and let it solidify, then repeat for every other seam, which would take forever. But it would let you use a clear epoxy.

If the primary objective is to let light through the seams, a translucent filler can work. Diffusers on light fixtures are often translucent to mask the ugly bulbs behind them or to spread and even out the light without significantly reducing the amount of light.

The difference between clear and translucent is that clear materials let light travel straight through the material, so you can see things like the edges of what's bordering the fill. Translucent materials let light pass through, but it is bent in random directions on the way, becoming diffused. The light is too jumbled to make out details, but it will pick up large-scale characteristics, like the reflected color of what's bordering the fill.

Clear grout works that way. These tend to be translucent more than clear, but pick up the color of the glass or tile it's in contact with. You can add clear filler to something like epoxy (fumed silica, which is basically glass, is often used). That thickens it so it stays where you put it and doesn't flow. But if the refractive index of the fill doesn't match that of the epoxy, light gets bent in random directions as it passes from one material to the next within the mixture, and it becomes translucent.

One thing to keep in mind on projects like this: you can usually find materials that will work. But often, the materials will be expensive, at least in the quantity you need, or are sold for an application that normally uses it on a different scale. You might need to buy a much larger quantity than you need as the smallest size available, or might need to buy wholesale quantities of something normally used in tiny amounts.

Here are a few ready-made materials that could work:

  • Clear grout. The 5 Best Transparent Grouts (2022 reviews) reviews some example products that the authors think are good (not all of the products listed are close to transparent, though, or else some of the links are corrupted and point to something else).

    The manufacturers appear to use "transparent" for any product that picks up the color of what it is in contact with. That effect only needs to happen near the surface, a thick layer can be close to opaque. Reviewers of several of the products on Amazon discussed hoping to illuminate the grout in one area and have the light spread through the seams so they glowed over a wide area, but the illumination was very local. If you have a strong light on the inside of your piece, you should get some amount of glow to highlight the wood, but it won't be super bright.

    You can largely control the surface at the time you apply it. You'll be able to sand off errant bits of grout, even smooth the surface if needed. You probably won't be able to effectively polish the surface to make it shiny, but that won't affect it's performance. These are translucent, and even a very smooth surface is likely to have a bit of a frosted look.

    A closely related product is "clear" epoxy repair medium designed for things like granite countertops and tile. Here's an example. The viscosity is low enough for it to act like a glue in a hairline crack, but it is thick enough to mostly stay where you put it. So if it contains a filler, there probably isn't much.

    In a repair, you cut off the excess after it hardens. The seam picks up the color of the surrounding material, making it close to invisible. Thin layers are clear, thick layers are somewhat opaque, but it should transmit enough of the light to at least glow and highlight the wood.

  • Clear UV resin. If you start with a thick (viscous) UV resin, it will pretty much stay where you put it until you cure it, and you can do one seam at a time so you aren't fighting gravity. You can also build it up in layers. These will typically flow enough to self-level, and they usually come in dispenser bottles so you can put it exactly where you want it, so you shouldn't need to polish the surface. However, you could. Dentists often use UV resin for temporary fillings, and grind and polish them. Some clear UV resins will eventually yellow, others don't. What is UV Glue? Properties, Pros and Cons, Best Brands has some good discussion and some product reviews. Besides the products sold primarily as glues, you can get product sold as resin for molding.

  • Moldable plastic. There is a type of plastic that you warm in hot water or with a low temperature heat gun and it becomes very soft and pliable so you can mold it. When it cools, it's hard and similar to nylon. It's clear when hot and becomes translucent when cold, but it should transmit light well in the thickness you're talking about (just use the uncolored material; you could also mix in some color pellets to tint it but I'm not aware of how that affects transparency). When hot, it will bond to many materials, including wood, a bit like hot melt glue.

    For your project, you would soften it and roll it into a rod, then press it into the seam. You can smooth the surface by warming it with a heat gun, then wetting your finger and lightly rubbing it to smooth and polish it. You shouldn't need to sand or polish it, but you could once it's cool (it's machinable). Just take your time, and maybe lubricate the surface with some water, so the friction doesn't build up enough heat to resoften it.

    The material doesn't become liquid and flow when hot, it just becomes soft and moldable. The working temperature is around 150°F, which can be hot enough to be uncomfortable to work with with your bare skin, although it softens a little below that temperature. The point being that enclosing a light inside the piece could build up heat if you don't use a source like LED light. I wouldn't worry with LED lights, but if a hot light source is used and it is on for a long time, this filler could soften to the point where someone handling the piece could deform the filler, in which case you would need to smooth it out again.

    A number of companies make the product, and they're all pretty equivalent. You can find them by doing a search like hot water moldable plastic.

  • Clear hot melt glue. Even normal, translucent hot melt glue will transmit light and work for this purpose. There are lots of YouTube videos of people making flashlights/reading lights or "light sabers" from a glue stick with the end melted onto an LED bulb. But you can get it in clear, which would be closer to what you're looking for. Much of what is readily available is multi-temperature, which means it melts at the low temperature setting and doesn't self-destruct at the high setting. The low temperature range is still pretty high in terms of what it would encounter under normal circumstances. If you can find high-temperature-only glue sticks, that would be better, but even low-temperature or multi-temperature should work.

    A good glue gun will let you control where you put it, and you can get it to self-level by going over it with a heat gun at a low temperature setting. If you're careful, there wouldn't be a need to sand and polish. Errant glue can be scraped off with a knife or removed from the wood with a little sanding. But it is a pretty soft material that gets softer or becomes liquid when it heats up, so it isn't practical to use sanding to polish the surface.

    You can mold a polished surface into it, though. For example, mold a flat or curved piece that fits the top of the seam from silicone caulk. Heat the glue surface to melt it and press the molded piece against it. When the glue is cold, peel off the mold and the hot glue surface will be smooth and shiny. You can also use other materials, like a glass or metal rod. A few drops of alcohol at the edge of the glue after it cools will wick in and release the form.

    Hot melt glue stays hard in a big enough temperature range that I wouldn't worry about it melting if you use LED lighting inside, especially if you use high-temperature glue. But if you create an "Easy Bake Oven" by enclosing a halogen or incandescent bulb inside, and leave it on for a long time, low-temperature or multi-temperature glue could start to melt. Even then, it probably wouldn't flow anywhere. But if it did get hot enough, the glue could become liquid and potentially flow and make a mess.

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    Oh, UV resin is a great idea. It has all the positive attributes of epoxy but avoids most of the negative ones. I'm actually disappointed at me that I didn't have the idea myself.
    – Elmy
    Commented Jul 28, 2022 at 11:39
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    I'm going to take a shot at UV resin on some of the less visible cracks. Will update with photo proof if it works.
    – Seph Reed
    Commented Jul 29, 2022 at 4:12
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I mainly write this answer to give you the possible pros and cons of using epoxy. Without seeing your project (mainly the size) I cannot recommend using it. You can achieve incredible results with epoxy, but honestly it's a difficult material.

You can find fast curing (5 - 10 minutes) epoxies in many construction stores or online. They typically look like this:

enter image description here
This is a random example. I'm not affiliated with the manufacturer in any way.

However, the hardener usually has a yellow tint and the cured epoxy will continue to yellow in time. Honestly it looks very old and unattraktive. You can create nice effects by tinting it red, though (read this related question). Trying to tint it blue will result in an ugly green tone.

If you want perfectly colorless and clear epoxy that won't yellow too much in time, you'll have to buy a special craft resin.

enter image description here
This is a random example. I'm not affiliated with the manufacturer in any way.

They are often marketed as "liquid glass", "jewelery" or "crystal" resins. However, these are usually meant to be poured and then level out on their own, so the consistency is very liquid. Different brands offer different curing times (usually between 30 minutes and 24 hours).

If you want to use such a liquid epoxy, you either need to apply many thin layers and let each cure completely, or you have to wait after mixing the components until it starts thickening up. Once the epoxy starts thickening, there's only a very short time left to work with it (until it's too hard) and you risk trapping air bubbles in it. If you plan on sanding the finished piece anyways, that might be acceptable for you.

Mixing ratios

You might get the idea that mixing more hardener into the epoxy should make it more viscous, but the opposite is true. Both components (resin and hardener) are initially liquid and must be mixed as exactly as possible in the ratio defined by the manufacturer. Having too much or not enough hardener will result in a sticky resin that never fully cures.

Fillers

Mixing crushed glass shards or tiny glass balls into epoxy creates a pretty effect. Glass has a different refraction index than epoxy, so you'll be able to see where glass and epoxy touch. With glass shards it looks like a shattered crystal but has the full strength of epoxy.

enter image description here
Image source. This is a random example. I'm not affiliated with the manufacturer in any way.

Health considerations

All epoxies must only be used outside or in a well ventilated room or you must wear a respirator the entire time while working with it. Even if the resin doesn't outright stink, the chemicals released while curing can irritate your airways and eyes and even cause allergies. You have to wear nitrile gloves because skin contact can cause allergies even several hours after contact.

You must wear a mask (like an N95 or FFP2 or better) whenever you sand epoxy and you should keep the sandpaper wet to bind the dust. The dust becomes airborne very quickly, gets inhaled and irritates your airways and lungs. Since Epoxy is anorganic, your body cannot break it down and it stays in your lungs.

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