Bubbles in epoxy: where they come from and how to stop them

Bubbles are a frequent complaint with clear epoxy, and they are mostly preventable. They come from three sources, and each source has a different fix. Working out which one you are fighting saves a lot of torching.

Source 1: air mixed in

Stirring folds air into the resin. EcoPoxy says entrapped air is usually caused by aggressive mixing, and advises mixing slowly until the epoxy is streak-free and clear (EcoPoxy).

Signs: many small bubbles spread evenly through the whole pour, visible right after mixing.

Fixes:

• Mix slowly and scrape the sides and bottom of the container.
• For deep pours, give the bubbles time to rise before pouring if the data sheet says so. TotalBoat’s Fathom data sheet, for example, says to let the mixed epoxy stand in the pail for 20 to 30 minutes so air bubbles can rise (TotalBoat Fathom TDS). Do this only when the data sheet says to: a fast coating resin left in the bucket heats up and shortens its working time.

Source 2: air escaping from wood and objects

Wood is full of air. When resin covers it, that air comes out. EcoPoxy explains that porous materials like wood, and curved objects like shells, hold air that moves into the resin after they are submerged (EcoPoxy).

Signs: bubbles that keep appearing along the wood edges or under embedded objects long after you poured, sometimes in trails.

Fixes:

• Seal coat first. EcoPoxy recommends seal-coating any surface that will be submerged in its casting resin, because air and moisture escaping from the material cause bubbles (EcoPoxy). TotalBoat describes the seal coat as an airtight barrier between porous wood and the flood coats, and applies it as a 1/16 in layer spread over the surface and into every crack and void (TotalBoat).
• Pour slowly around objects. EcoPoxy notes that pouring too quickly traps pockets of air under objects, which can migrate out during the cure (EcoPoxy).
• Pre-wet curved objects. EcoPoxy suggests brushing objects with curved surfaces with resin before placing them, to break the surface tension (EcoPoxy).

Source 3: resin that is too cold

Cold resin is thick, and thick resin holds bubbles. ArtResin says cold resin thickens and can turn cloudy or milky, because very small bubbles stay trapped in it (ArtResin).

Signs: a hazy, milky look rather than distinct bubbles; resin that pours slowly.

Fixes:

• Bring the resin, the hardener and the workpiece to the temperature range on the data sheet before mixing. The temperature and timing helper compares your shop with the published range.
• ArtResin’s suggestion for its own product, when in a hurry, is to stand the closed bottles in warm water for roughly 10 minutes, then dry them completely before opening, noting that warming shortens the working time (ArtResin). Check your own product’s instructions before warming; faster resins react more strongly to heat.

Popping bubbles that rise

For bubbles that reach the surface during the working time, manufacturers describe the same tools:

• Torch. EcoPoxy’s instructions are to hold the flame tip 1 to 2 in off the surface, move quickly in a sweeping motion, and never let the flame touch the epoxy (EcoPoxy).
• Heat gun. EcoPoxy says a heat gun also works, but care is needed not to disturb the resin surface (EcoPoxy).
• Pick. A toothpick or other pointed tool can release a single stubborn bubble (EcoPoxy).

Over-torching is a real risk. ArtResin states that too much heat in one spot can cause rippling, scorching or uneven curing, and advises smooth, steady passes without hovering (ArtResin). Stop once the surface is clear. Keep flames away from solvents, tape backings and dust, and follow the product’s safety data sheet on ventilation.

Diagnosing a cured piece

What you see	Likely source	Next time
Even spread of tiny bubbles	Mixing	Mix slower; follow the data sheet on standing time
Bubbles along wood edges or under objects	Air from the wood	Seal coat; pour slowly around objects
Milky or cloudy haze	Cold resin	Warm the room and materials into the published range
Rippled or scorched spots	Too much torch heat in one spot	Keep the torch moving

If the bubbles are near the surface of a coating, a second coat is the usual repair. ArtResin, for example, says a second coat can be used to remove bubbles, dust or hair, poured over the first once it has cured and been lightly sanded (ArtResin); follow your own product’s recoat instructions. A second coat cannot reach bubbles deep in a thick pour.