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Are there any techniques that can be used to imitate an oily sheen on a surface using matt acrylic paints and an airbrush?

I want to color the carapace on a large beetle shell that ranges from black\green to light blue\green, with the light coming from above. I have only matt acrylic paints and want to use them if possible rather than buy pearlescent paints that I may never use again, and to explore new techniques.

My plan B is to do a textbook zenithal highlight, and then use a gloss varnish.

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  • Are you looking to render the look or hoping to recreate the optical effect? That is are you painting in the prismatic highlights or trying to get the highlights to happen dynamically.
    – rebusB
    Aug 28, 2023 at 17:55
  • I'm not sufficiently familiar with the words you are using to understand the difference. I want to approximate the effect of a surface being much more reflective than it really is, using matt paints, because I only have matt paints. Aug 28, 2023 at 18:45

3 Answers 3

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So you want something like this, I take it?

enter image description here
source

Since the colours are mostly defined by the reflection, your requirement of the light source being above the beetle doesn't say much about how the colours on the beetle are perceived, so I'll assume it is also observed from above.

The typical colours are caused by iridescence, which makes it so colours change depending on the viewing angle and angle of incidence of light.
Note that what colours you see are those of the sky reflected (in slightly altered hues)—the dark parts don't reflect enough light to change the colour, and you can see a reflection of what are possibly trees on the sides of the dung beetle. If the carapace will remain in a single area, take this into account, and imagine how the reflections change based on that area (e.g. in front of a window looking outside, the blue hue would be visible towards the lit side of the carapace).

What will work for a static effect is to paint the carapace black, then spray-paint a (greenish) blue gradient towards the centre or zenith of the carapace. Smoother areas can be spray-painted further away from the surface, and irregular surfaces might require more precise and closer spraying, and possibly manual brush work.
You can imitate a striated structure like the one seen in the image above by leaving a few stripes black. What will make the effect pop is to have a more saturated blue colour towards one side of the shield, and a saturated green towards another (you can see this most clearly along the 'collar' of the beetle, and other detailed areas).

Afterwards, I'd still recommend using a gloss varnish to cover it all.

If the varnish is really glossy, you might even consider painting the carapace black and only applying a glossy varnish afterwards, so the reflections will actually change in real-time.

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In my opinion creating a "shiny" effect with matte paints will always look odd. The human eye is very keen on picking up tiny clues and drawing assumptions from them, so when we see a matte finish we expect the light to act typically for matte objects.

If we take for example a sphere with a single light source, the size and sharpness of the highlight created by the light source tells us about the properties of the surface. A small, sharp highlight with a sharp contrast between light and background implies a shiny, almost mirror-like surface. A blurry, spread-out highlight with a smooth transition between light and background implies a matte, maybe even rough surface.

enter image description here

When you try to recreate a shiny surface on an object with matte paint, you have to paint a lot of sharp details, shadows and highlights. However, this may evoke a completely different effect of looking blocky, maybe unrefined or just "off". Our eyes still see the natural shadows and highlights of the natural light which imply a matte surface and the painted shiny effect doesn't reconcile with what we see. I think this is like the uncanny valley for surface textures.

I have some experience with painting tabletop miniatures and weapons like swords and axes never looked right because the "silver" or "metal" paints have a grainy texture that evokes a matte impression. No matter how diligent I copied highlights and shadows from photos of real swords or other miniatures, my painted weapons always looked fake.

Here's an example I found on Reddit with beatifully painted weapons, but none of them look truely shiny to me. The green skin tome OTOH looks very shiny to me because the natural light bounces off of it like it would off a shiny surface - probably because it actually is shiny. So I highly recommend giving all surfaces that are supposed to be shiny an actual shiny finish.

enter image description here

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I think the effect you are looking for is from thin film interface, like a bubble has those swirling colors as light is refracting through the thin film of the bubble surface in different ways because of microscopic differences in thickness. It can be recreated with special paints, often called dichromatic or opal paints (opals have the same effect) that to do the same thing with light passing through it. I would call those dynamic because they change as the light and viewing angles change.

The only way to capture this effect with standard paints would be to reproduce the effect of the light refraction at chosen angle. You would recreate the hues and patterns of the highlights with static colors. The problem is this would be fixed, not dynamic, so it would NOT be the same as the original effect. It would be a snapshot of it. This works better with 2d painting since we do not expect it to change, we only see the object at one angle. On a three dimensional model it may not be as effective as using the special paints.

That being said, dichromatic acrylic paints are readily available. They are very transparent so they would work well as a layer over your base color. For painting a 3d model it would probably be worth trying them out as it would be a much more natural looking and dynamic finish.

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