Expert Epoxy Casting Tips – Part 2: Mastering Project Processing for Perfect Results

Have you also admired those massive hand-made epoxy river tables? Maybe something smaller, like a serving tray, knife holder, or lamp?

Honestly, I was sold since the first YouTube video I saw years ago. The epoxy casting process might need a bit of preparation and know-how, however, the results are amazing and definitely worth it. Let’s go through it together and learn useful tricks to make it as easy as possible.

In the previous article, we went through the process from the preparation phase to the epoxy mixing and casting. You have waited a few days as the mold has hardened. Carefully released it from the form, but wonder what the next steps are?

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Index Part 1 – Project Preparation

1. What is the size of your project?

2. What epoxy volume will you need?

3. Epoxy selection

4. Casting preparation

5. Making casting form/mold

6. Mixing

7. Casting

Index Part 2 – Project processing

1. Rough mold processing

2. Cut to size

3. Sanding

4. Repairs

5. Surface finish

Index Part 3 – Lessons learned

1. Mixing

2. Bubbles

3. Leaks

4. Bending

5. Shelf life

6. Tooling

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1. Rough mold processing

After a mold is released from the form, there is a time for the first flattening step. That is usually done via routing or rough grinding. Depending on the project, you have the following options:

For smaller projects like epoxy inlays or crack filling where base wooden material stayed flat and relatively low epoxy volume was applied, these tools are to be used to remove excessive hardened resin:

• Belt or orbital hand-held power sander
  Higher sand paper grids (60-80grt) are to be used. Friction generates heat. Make sure not to grind small areas for too long otherwise, epoxy color and structural degradation will appear.
  The usage of sandpaper designed for epoxy is a game changer. With cheap materials work takes forever.
  Do not forget good working place ventilation and, if possible, appropriate dust extraction. Personally, even with dust exhaust, I still wear a protective face mask.

• Hand power plainer or stationery plainer/jointer
  It can be combined with grinding. Local overheating is not an issue. Just watch for epoxy chipping if the cutting blades are not sharp enough.

Larger projects with more epoxy surfaces e.g., serving trays or river tables are a little more demanding. Two basic options are available in this case:

• Flattening jig and hand router
  Most common among hobby craftsmen. The flattening device is generally a horizontal bed with two straight and leveled bars on its sides. Over them drives a trolley holding a hand router with a large flat bit. Jigs can be self-made to respect the needed project size or purchased from several companies.
  I use a robust router trolley in combination with the self-made working bed made out of standard aluminum profiles.

  

• CNC router
  The “professional” way is to use CNC for almost perfect surface flatness. You do not need to own one yourself. Just search for woodworking companies in your area and give them a call. Most of them are willing to provide such a service for a reasonable cost.

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2. Cut to size

At this point, you have in your hands a perfectly flat mold. Yes, yet there are either scratches from high-grit grinding or swivel marks caused by routing. Those will be handled later. Now, your project is ready to be cut to the requested size.

Cutting can is usually done by:

• Table saw or track saw

• or CNC for more complex shapes

Use cutting blades with a higher tooth count to prevent edge chipping.

If you plan on having beveled or rounded edges on your project, now is the best time. Edges are mostly formed by hand routers. When the surface is finely sanded, routing is going to definitely cause additional new scratches, and the whole fine-grinding procedure has to be repeated.

To have fine, clean edges, use sharp, high-quality routing bits. To me, it also helped increase the router’s rpm. You acquired some scrap pieces a moment ago from the cut-to-size step. Try out edge routing on them first to ensure you have everything set right.

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3. Sanding

Honestly, this is my least favorite process step. However, it will become almost acceptable with proper tooling, setup, and skills. Tools used for grinding of the roughly flattened project are:

• Orbital sander
  Of course, you can decide to sand the whole project manually. However, in this case, time-saving and quality enhancement using the correct power tool is huge. I consider an orbital sander a must.
  Sander disk diameter is typically 125mm or 150mm. You will acknowledge the benefits of a bigger one, especially on larger surfaces.
  On the market are also products with driven sanding disc mode (or turbo mode). This improves tool efficiency in the early stages (lower grits). For fine high gloss epoxy sanding it shall not be used.

  

• Sandpaper set
  There are sandpaper manufacturers with products optimized for epoxy. I strongly suggest investing a little bit more in those products. With cheap papers, the work is extremely difficult and time-consuming. Trust me, I have learned it the hard way at the very beginning.

  These manufacturers provide proposed grit sequences for the best results. From my experience, improvising is not the best idea.

  

• Polishing machine & paste
  In case you intend to go for a high gloss finish, a polisher is needed, including appropriate pads and paste

  

• Dust exhaust
  Health safety rules for working in dusty environments shall be followed twice more fiercely when sanding epoxy. Moreover, if sanding to very fine grits. Ensure your workplace is well-ventilated and the dust extraction system works well. Personally, even with dust exhaust, I still wear a protective face mask.

What is your desire? Matt or glossy epoxy surface? I find staying with a matt surface much more practical, which means finishing sanding at 320-400grit. A mirror-like glossy surface requires sanding up to 3000grit and finishing with a polishing paste.

With glossy surfaces, you have to also consider a few drawbacks, and it seems to me nobody tends to mention them:

• All the dust particles and fingerprints are going to be visible

• All potential impurities and microbubbles inside the transparent epoxy cast will be noticeable

• Epoxy hardness is nowhere near to glass. Over time, your project will catch more and more scratches, which are again better to see on glossy surfaces.

• More time, equipment, and skill demanding. You have to work up through additional sandpaper grit levels, finishing with a polisher. If, by chance, you realize a scratch from previous levels was missed, you have to go back by one or more grit levels and work it up again.

Anyway, for some projects, a glossy finish looks awesome and can be achieved. Effects like 3D casting actually would not shine without a glossy surface. Just for a heavily used dining table, it does not make much sense to me.

The sanding process itself is then pretty straightforward (and boring)

1. Prepare your set of sanding papers and order it by grit. Keep your sandpapers in a clean spot. Think about where you put your sander off. Any workshop dirt, especially on fine ones, will cause unplanned scratches, resulting in additional work.

2. Set up dust exhaust system.

3. Start with the lowest one (usually 80, maybe 100grt).

4. Slowly sand the whole surface in a vertical and then horizontal direction. Make sure the sander disk you hold is totally flat. Remove all the scratches and swirls from the previous processing step. Make sure you keep sanding papers clean without dust build-ups. If you notice swirl patterns on the surface, stop and clean the sandpaper and surface of dust.

5. Carefully vacuum the rest and dust off.

6. Continue with the next grit. Do not skip the grit sequence! Go to step 4…

7. Repeat till you reach your maximum intended sanding level..

Polishing takes place as a final step when you aim for a glossy finish. Clean the surface very carefully with a micro-towel. Any particles left might cause additional scratches. Set your polisher to low rpm. I use a one-step nano polishing paste with good results. Take your time; do not hurry.

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Repairs

Fine flaws in the epoxy cast are mostly visible after initial sanding (100-200grit). The sanding process has to be paused here till necessary repairs are done.

The most common flaws for epoxy casting projects are:

• Wood cracks/imperfections found after material flattening

• Air bubbles/micro-bubbles in epoxy

• Foreign particles in epoxy

The impurities inside hardened epoxy volume are nearly impossible to correct. That is multiplied by transparent resin casts, where you see through the whole epoxy thickness. However, for surface flaws on wood or epoxy colored by inks or powders, there are several options:

• Mix a small volume of epoxy (preferably the faster hardening variant) and tone it the same as the base resin cast. It has better matching but it takes at least 12-24 hours.

• CA glue is used in combination with an activator for small flaws like bubbles. Mix a few drops aside and tone them. On the needle tip, let it drop into the bubble void. Apply the activator afterward for instant hardening. This way is much faster. Just keep in mind that CA glue itself has higher hardness and a slightly different light refraction index.

  

• In some cases, repairs even of colored resin casts can be done with a clear transparent mixture, also with very good results.

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Surface finish

The final surface treatment is about magnifying the impression of your project or ruining it. Most of the makers want to increase the contrast between epoxy and wood grains.

• Hard wax oils 
  The most common finish is by using a variety of hard wax oils. The epoxy pattern, as well as grain structure, amazingly pops up, and the surface is very pleasant to touch.

  • The application is simple, fast, and in one or two layers.

  • Follow the instructions on the chosen product. Stay patient even if it might be difficult, and respect hardening times.

  • Buffing by a scotch brite (fine/very fine) in combination with a low-speed directly driven orbital sander, from my own experience, saves a lot of time and increases quality.

  • Many of them are also friendly to potential future small surface repairs.

  • However, you have to respect somehow limited surface protection compared to, e.g., polyurethane coatings

  • Some manufacturers also restrict wood sanding roughness because of the optimal oil soaking into the grains, e.g., 220-320grit, which might go against the high gloss epoxy sanding process. In case you tend to ignore the suggested surface roughness, make a test on a scrap piece.

  • The most popular brands are:

    • Rubio Monocoat – pretty expensive, but can be applied in one layer. Precolouring or aging complementary products extends your options even further. In my experience, it is also the easiest and fastest application.

      

    • OSMO – another high-quality finish. At least two coatings are suggested. The color palette is quite limited to base tones.

      

• Flood epoxy coating
  For some cases, you might want to try flood coating, which means applying a final epoxy level on top of our project without any additional sanding.

  • Base roughness shall be about 220 grit

  • Carefully clean the sanded surfaces

  • Pour a thin epoxy layer

  • Let it spread evenly and allow the resin excess to drop off over vertical edges

  • After it hardens, just sand the bottom side.

  • I do not prefer this option, as I have never been able to achieve perfectly flat surfaces.

• Polyuretane coatings
  It is best applied with a paint gun. Multiple layers and sanding are needed. Provides very good surface protection. Perfect if your aim is a glossy surface of the whole project.

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Looking for more crafting tips and tricks?

…to be continued in Part 3: Lessons learned (and sometimes hard way)