1

For this kind of project I have in mind using epoxy of different viscosity:

  • to create the "base structure" use (very) viscous epoxy - especially for the curved area; I could shape it like I could shape cement;
  • to create a more beautiful finish - use (very) fluid epoxy.

Of course (especially considering this answer), the epoxy should have the same chemicals, but be "adjustable" to be thinner or thicker.

Ideally, the agent should not change the chemistry (much).

I guess that the curing time is not an issue - unless it extends to days / weeks / more.

0
2

Some epoxies are more fluid at higher temperatures, at least at first. The higher temperature also causes faster cooling. I've used this to good effect with epoxy adhesives. A slow-setting (several hours to give a useful bond) product warmed up to about 40°C gives a very good bond as it wets the surfaces well; it then cures fairly quickly, though not as quickly as the equivalent fast product. Conversely chilling should both thicken and slow down the reaction. Using a fairly liquid version and chilling for the thicker step might be worth a try, however it will still slump unless restrained by a mould.

Additives are likely to be problematic for thinning: they'll be liquids that are likely to want to evaporate, so adding too much will lead to significant shrinkage; they'll also affect the cure. Water is a bad idea - it will lead to a cloudy and weak result, but small amounts of acetone or denatured alcohol can apparently be used if you want a very thin layer. Mixing isn't easy, which means it's likely to introduce bubbles (as it did when I tried using alcohol-soluble dies. So this probably isn't an option for forming or casting, but may help your topcoat.

Because formulations are proprietary, you'll have to do some experimenting on a smaller scale, and hope that translates well to a large scale. Other people may have used specific products that you can easily buy, and their results might help you get started.

2

You may be giving yourself unnecessary constraints that would make it harder to achieve the result. You need a structural layer that will stay where you put it during creation, and a finish layer that will bond with it and flow enough to self-level into a smooth surface. The structural layer needs to cure into the right characteristics for strength and not be brittle. The finish layer needs to cure with different characteristics to achieve a different purpose. Trying to achieve the two different purposes by just adjusting the viscosity of the same epoxy isn't a good way to do it.

Structural layer

Creation stage: Anything that's just a viscous liquid will flow. That's a problem when you're working against gravity with non-horizontal surfaces; even horizontal surfaces if you don't constrain the edges.

There are a couple of ways to overcome that. One is to use a material designed to not flow. There are epoxy putties and clays that are designed to be worked like clay. You can form it into what you want and it will stay like that. So you could press it against your form and work it into a flat surface of the desired thickness, or roll it into a flat sheet of the desired thickness, lay it onto the form and trim the excess. That might work well with your idea of making sections and then joining them.

Another is epoxy paste, like Bondo, designed for purposes like repairing car bodies. It can be spread and shaped and will stay as you put it. You could just spread a layer of the desired thickness on your form.

A different way to avoid slumping is to work in layers, impregnating liquid epoxy into something that holds the epoxy in place. You can use something like nylon or fiberglass mesh (similar to what's used to strengthen repairs of cracks in drywall or concrete), or a fabric. You make each layer not much thicker than the mesh or fabric, so there isn't free epoxy to flow.

Strength: Epoxy tends to be brittle. Some epoxy clay or paste can be even more so; they contain fillers that may be optimized for purposes other than strength (there are also clays and pastes that contain reinforcing material, so it's stronger than plain resin). If you were to cast a thick piece, it would have plenty of strength. But creating a relatively thin sheet for the shell won't. One way to strengthen it is to reinforce it.

If you use epoxy clay, you can embed mesh into it, or create two thinner layers and sandwich the mesh between them. If you use layers of liquid resin with mesh or a non-stretchable fabric like fiberglass, that provides the reinforcement. A relatively thin skin of resin and fiberglass is strong enough for purposes like car and boat bodies. If you use a product that contains reinforcing fibers (like certain versions of Bondo), you wouldn't need additional reinforcement; just figure out how thick the shell needs to be for the required strength and rigidity.

Finish layer

The structural epoxy shell doesn't necessarily need a finish layer. You can think of the whole shell as a thick finish; it just doesn't have a super-smooth surface. If you color the epoxy used for the shell, you could just smooth and polish the surface. The only reasons to add a finish layer are for the color or to save some of the work of polishing the surface.

If you make the shell in sections and join them, you can polish each section before you join them. If you apply a finish layer, do that to the entire unit after the sections are joined. You would likely use epoxy clay or paste in the joints, and it's better to bond that with the shell material than to a finish layer.

It isn't really a problem getting finishes to bond with epoxy. You can choose a finish material that's the best for the job. There isn't a good reason to use a low-viscosity epoxy for that. You can pour resin over a flat surface and get a nice finish, but that won't work on non-horizontal surfaces; it will flow.

To finish a non-horizontal surface, you need a material that acts like paint. You need to be able to apply it in thin layers that flow enough to blend and self-level locally into a smooth surface, and quickly harden enough to not flow more. Epoxy resin is terrible for that purpose; it has all the wrong characteristics. You can do it in an industrial setting, but it isn't practical otherwise.

There isn't a need to reinvent the wheel. Applying a tough, nice finish to epoxy has already been perfected, is done routinely, and is readily available. It's the finishing step of car body repair. You can get automotive paint in any color, and clear coat, in handy spray cans. That will do a much better job, more simply, than trying to apply epoxy resin uniformly to big 3D pieces.

1

A modeling epoxy does exactly what you want for both the structural and the surface finish you want and can be acquired in large amounts.

This is the same kind of epoxy used in the recently popular river tables. You can pour it in thick layers (or all at once) into your mold, let it cure, remove your mold, sand and somewhat polish it, then pour on another thin layer to get the slick "wet look".

Many epoxies can be colored/tinted, which is what they do for river tables. It comes clear, then they use tints/dyes/powders/pigments to make it the color they want. If you wanted to get fancy, you could do multiple pours of epoxy with multiple colors. If you do them at the same time, you might get a marbles look. Waiting until the epoxy is set in between each pour will get sudden transitions. Then you leave the final/finish coat as clear to see what you created.

Epoxies like this generally come with instructions on how to pour epoxy on top of itself, so follow those instructions to get good adhesion between layers.

2
  • Take a look at the project link in the question. It's a working double sink. You can cast almost any shape, but this needs provisions for the overflow plumbing, etc., and the basins are deeper than the top. Even casting this in sections would be extremely complex and require great precision. Then almost none of it is horizontal, so you can't pour a finish layer like you could for something like a tabletop. – fixer1234 Feb 14 '20 at 23:19
  • @fixer1234, I looked at the project and it looks doable to me. And yes, I've used epoxy finish coats on vertical surfaces. Of course, this isn't something that should be done as a first project, since it is really complicated, as you say, but that doesn't mean it's impossible. If I had more experience with epoxies, I might get into making multicolored epoxy resin sinks. It sounds pretty awesome. – computercarguy Feb 14 '20 at 23:26
1

I'll add another answer that may more directly address what you asked, and is very relevant to computercarguy's answer.

This question sounds like you're contemplating an approach to create a reasonably thin shell that follows the sink contours. You can do that either as a thick coating or series of layers on a form, or cast it in a two-part mold that constrains the thickness. A different approach would be to create a form for the cavity (the shape of the hole), put that inside a big rectangular form, and cast a big block that has the right shape on what becomes the visible side.

The question describes using a viscous epoxy for the structural shell so it better stays where you put it, and a non-viscous one for a finish coat, possibly by adjusting the viscosity of the same epoxy.

As Chris H's answer describes, you can adjust the viscosity of epoxy somewhat, but it affects other characteristics, so that may not achieve the result you want. For example, if you chill it, you may slow down its flow, but you also slow down the cure. It will slump slower but also cure slower. So you will still end up with it slumping; the whole process will just take longer. Thinning or thickening by adding other materials introduces other side effects or changes in characteristics.

Instead of trying to change the viscosity, you can just use epoxies that already have the viscosity you need (or are best for the application). There are resin systems designed for casting, and ones designed for finish coating (and there are compatible ones so you can use them together). The casting resins are designed for layers up to several inches thick. The finishing resins are typically designed for layers less than about 1/4" thick.

Each is optimized for its purpose and there are problems trying to use them for the opposite purpose. The viscosities are actually the opposite of what's described in the question.

Casting resin tends to be low viscosity, often close to water, with a slow cure rate. When you apply a finish coat to something like a tabletop (or sink), the process is typically to allow the resin to flow down the sides and off (wasting some). Casting resin is generally too thin for that purpose; most of it would run off. And because casting resin takes longer to cure, it increases the risk of getting dust (or bugs) stuck in it.

The resin designed for finish-coating tabletops is very viscous, which produces a thicker layer and limits how much runs off and is wasted. It is also designed to cure quickly, which generates a lot of heat. If you try to cast something more than about 1/4" thick, the heat it generates destroys the epoxy.

There's a good discussion of the differences here: Casting Resin Vs Table Top Epoxy Resin. It has a short embedded summary video (direct link here), that will save some reading time.

If you want to create a shell by coating a form, as described in the question, it would probably be best to do everything (structural shell and finish layer) with the viscous material designed for finishing tabletops. You can build it up in layers.

If you wanted to try making it as a large casting using just a cavity mold, use low-viscosity casting resin for the structure. With a good mold, you wouldn't need a finish layer. But it the surface requires cleanup and you want to add a finish layer, use the high-viscosity resin designed for that purpose.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.