Bubbles

Stirred in

Vigorous mixing whips air into the resin. Slow, deliberate stirring that keeps the stick below the surface puts far less in, and it costs nothing. This is the largest source for most makers and the easiest to fix.

Out of the substrate

Porous material, wood especially, holds air that the resin warms and drives out into the casting. Sealing the surface with a thin coat first, and letting it cure, stops the bulk pour from having to deal with it.

Out of solution

Dissolved air comes out as the mix warms during cure, which is why bubbles can appear in a casting that looked clear when poured. Warming the resin slightly before mixing lowers its viscosity and lets bubbles rise and escape before it gels.

Heat at the surface, carefully

A brief pass with a torch or heat gun pops surface bubbles by thinning the resin locally. Overdoing it adds heat to a system that makes its own, and scorching or accelerating the cure is a real risk, so brief and moving is the technique.

Pressure and vacuum do different jobs

A pressure pot crushes bubbles small enough to be invisible and is the usual answer for castings. A vacuum chamber pulls them out before pouring and suits silicone molds. They are not interchangeable and buying the wrong one is a common expensive mistake.

Decide whether it matters

A river table needs clarity; a painted or opaque piece does not. Investing in pressure equipment for work where nobody will see the bubbles is a cost with no return.

Pour slowly and from low down

A thin stream falling a long way folds air into the mold on the way in, and no amount of torching afterwards fully recovers it. Keeping the cup close to the surface and pouring into one spot costs nothing and removes a share of the problem before it starts.

References