Short answer: warm the bottles in warm water before mixing, stir slowly, pour thin, and pass a heat gun or torch quickly over the surface a few minutes after pouring. Bubbles that are already cured have to be sanded out and covered with a fresh coat.
Where bubbles come from
Mixing: fast whipping folds air in.
Cold resin: thick resin traps air. Resin at about 75°F releases bubbles far more easily.
Porous materials: wood, cork, paper and dried flowers release air as resin soaks in. This is why bubbles keep appearing after you torch.
Deep pours: slow-curing casting resins release bubbles on their own, but thick fast resins trap them.
Step by step
Set the sealed bottles in warm (not hot) water for 5–10 minutes.
Mix slowly, scraping the sides and bottom, for the time on the label (often 3 minutes).
Let the cup rest a minute so large bubbles rise, then pour low and slow.
Seal wood or porous inserts first with a thin scraped coat.
After 5–15 minutes, sweep a heat gun or torch over the surface, a few inches away and moving. Repeat once or twice in the first 30–45 minutes.
For small castings, a pressure pot (around 40–60 psi) shrinks the remaining bubbles until they are invisible.
Heat gun vs torch vs alcohol spray
A torch works fastest on flat coats. A heat gun is gentler and safer on silicone molds and tumblers. A fine mist of isopropyl alcohol is sometimes used to pop surface bubbles, but too much can leave marks or affect the surface, so test first. Too much heat in any form can ripple the surface, scorch it or speed up curing, so keep moving.
FAQ
How do you get bubbles out of resin after it cures?
You cannot pop cured bubbles. Sand the surface (around 220–320 grit), wipe off the dust, fill open holes with a little resin and add a thin new coat.
Why does my resin keep getting bubbles after I torch it?
Usually air is escaping from wood or other porous material underneath, or the resin was cold. Seal-coat porous surfaces first and warm the resin before mixing.
Can I use a hair dryer to get bubbles out of resin?
A hair dryer is weaker than a heat gun and blows air that can push resin and dust around. It may help on thin coats but a heat gun or torch works better.
Should I torch resin in a silicone mold?
Use a heat gun or very quick torch passes. Lingering flame can damage silicone and overheat thin resin.