I have a project where I want epoxy resin in a solid opaque white rather than clear or tinted.
The obvious thing to hand is white acrylic paint, and I wondered whether stirring some into the resin before it cures would work, or whether that causes problems.
If it is a bad idea I would rather know before mixing a batch I cannot undo.
It is a bad idea, and the reason is short: acrylic paint is water-based, and water interferes with epoxy curing.
Epoxy cures by a chemical reaction between resin and hardener in fixed proportions. Introducing water — and acrylic paint is largely water plus binder — can leave you with soft spots, a tacky surface, cloudiness, or in bad cases a batch that never fully hardens. Small amounts sometimes get away with it, which is exactly why the advice gets ignored until somebody loses a piece.
What you actually want is the pigment without the water: titanium dioxide powder, which is the standard white pigment used in almost everything, including the acrylic paint you were about to use. Adding the powder directly gives you the colour and leaves out the vehicle.
Very little is needed — start with a fraction of a percent by weight and add gradually. Pigment powders are far stronger than people expect.
Practical safety point since powders are involved: dry pigment is very fine and easy to inhale, and titanium dioxide dust is not something to breathe casually.
Mix it in a way that does not raise a cloud — add it into the liquid rather than the other way round, work with the container low, and wear a dust mask while handling the powder. Once it is in the resin it is bound and harmless.
Same goes for the resin itself: gloves, ventilation, and no skin contact. Epoxy sensitisation is cumulative and people who get careless with it early tend to regret it.
One thing to be aware of with titanium dioxide specifically: it is an extremely effective opacifier, so it is easy to overshoot into something that looks chalky rather than solid.
If you want a white that reads as a material rather than as paint, a very small amount of a second pigment often helps — a trace of blue or grey stops it looking yellowish, which pure white epoxy tends towards as it ages.
Also worth knowing that epoxy yellows over time with light exposure unless it is a formulation with ultraviolet stabilisers. A white piece shows that far more than a coloured one, so if the project will sit in daylight it is worth choosing a resin sold as non-yellowing.
@propmaker_pru · last wk.
The general rule for colouring epoxy: use a colourant that is compatible with the system, and keep the total addition small.
The options that work:
What does not work reliably: anything water-based, and anything added in large volume. Even a compatible colourant will weaken the cure if you add enough of it — the usual guidance is to keep additions to a small single-digit percentage of the total mass.
And mix the resin and hardener thoroughly first, then add pigment. Adding pigment to one component before combining makes it much harder to judge whether the two parts are properly mixed, and under-mixed epoxy is the other common way to get a batch that stays soft.
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