Why Resin Has Pinholes — and How to Prevent Them

You demold a resin piece, turn it over, and there they are: a scatter of tiny pinholes across an otherwise glossy face. Surface holes are the most common flaw in resin casting — and the most preventable. The trick is knowing that where the hole sits tells you exactly what went wrong. This guide covers the three sources of surface bubbles, the prevention routine that stops them, and how to rescue a cured piece that already has them.

Why does cured resin have holes or bubbles on the surface?

Surface holes come from three places: air whipped in during mixing, air trapped against the mold's walls and detail, and bubbles that rise and burst late in the cure. Each leaves a different fingerprint, so diagnose before you change your process.

The mold-facing pinholes are the ones that sting most, because you don't see them until demolding. The good news: they're also the easiest to eliminate once you know the routine.

How do you prevent bubbles before you pour?

Warm your resin, mix slowly, and let the mixed cup rest before pouring — thin, warm resin releases air on its own. Cold resin is thick, and thick resin holds bubbles like honey. This one habit fixes more surface flaws than any other.

1. Warm the bottles in a bath of warm (not hot) water for 5–10 minutes before measuring. The resin should flow like light syrup.
2. Mix gently for 3 minutes, stirring under the surface and scraping the sides — whipping is how air gets in.
3. Let the mixed cup rest: about 5 minutes for a shallow pour, 15–20 minutes for a deep one, so bubbles rise and pop in the cup instead of the mold. Mind your resin's working time.
4. Keep the room at 21–24°C. A cold workspace thickens resin and slows the bubbles' escape.

If your piece is tacky rather than holey, that's a different problem — a mixing-ratio issue, covered in our guide on resin mix ratios.

How do you stop pinholes on the mold-facing surface?

Coat the mold's detail with a thin layer of resin before the main pour, then tap and pinch the walls as you fill. Pinholes on the cast's face mean air clung to the silicone and never escaped — so your job is to wet every crevice before the bulk of the resin arrives.

1. Brush or drizzle a thin first coat into textures, corners, and fine detail, working it in with a stick or soft brush.
2. Pour the main fill slowly in a thin stream from one corner, letting resin flow across the cavity.
3. Tap the mold on the table and gently pinch or flex the outer walls so bubbles clinging to the silicone break loose and rise.

Detailed and faceted designs reward this routine most — the crisp geometry in our resin molds comes out glass-perfect when the first coat goes in, and freckled when it doesn't. I learned it on a faceted piece that emerged with a rash of pinholes across every facet; one brushed coat later, the next cast was flawless.

How do you remove bubbles after pouring?

Sweep a torch or heat gun over the surface in quick passes, then keep an eye on the piece for the first hour. Heat thins the top layer so risen bubbles pop, but bubbles keep surfacing as the resin settles — so one pass isn't enough.

1. Torch in fast sweeps a few inches above the surface, never lingering — scorched resin yellows.
2. Pop stragglers with a toothpick or a light mist of isopropyl alcohol, which breaks surface tension.
3. Check back every 15 minutes for the first hour. Late-rising bubbles can be popped right up until the resin gels.

What about a pressure pot — do you need one?

No for most craft work; yes if you sell flawless deep casts. A pressure pot cures the resin under pressure, compressing any remaining bubbles to invisibility — it's how professionals get glass-clear spheres and thick jewelry blanks with zero flaws. For coasters, trays, and shallow decorative pieces, the warm-rest-torch routine gets you 99% of the way without the equipment.

If you're casting thick geometric pieces like our deep-pour geometric mold, use a slow deep-pour resin and the long 15–20 minute rest — deep casts give bubbles more time to escape naturally, which is the budget alternative to pressure.

Does humidity cause surface problems too?

Yes — moisture causes cloudiness and a dimpled, greasy-looking surface called amine blush, which is different from air bubbles. Epoxy hates water in every form: a damp mold, a humid room, even condensation from cold bottles warmed too fast. Keep your molds bone dry, work below roughly 60% humidity when you can, and dry the mold thoroughly after washing. If your flaw looks like fog or film rather than holes, moisture is the suspect.

Can you fix holes in already-cured resin?

Yes — fill the holes with fresh resin, or sand and recoat the whole face. A cured piece with pinholes isn't ruined; it just needs a second session.

1. For a few scattered pinholes: drop in a bead of UV resin or freshly mixed epoxy with a toothpick, let it self-level, and cure. UV resin makes this a 5-minute fix.
2. For a peppered surface: wet-sand at 400 grit, then 600, then 1000 until uniformly matte, wipe clean, and pour or brush a thin flood coat over the face. It cures back to full gloss.
3. For deep voids: fill in layers, letting each cure, then finish with the sand-and-coat routine.

Flat pieces like coasters are the easiest to rescue this way — the full finishing method is in our guide to making resin coasters.

The no-pinhole routine, start to finish

Here's the whole sequence in one place — pin it above your workbench.

1. Warm the bottles 5–10 minutes in warm water.
2. Mix slowly for 3 minutes; rest the cup 5–20 minutes by pour depth.
3. Brush a thin first coat into the mold's detail.
4. Pour low and slow from one corner; tap and pinch the walls.
5. Torch in quick sweeps; babysit the first hour with a toothpick.
6. Cure warm, dry, covered, and undisturbed.

Frequently Asked Questions

Why does my resin have tiny holes on the surface?

Tiny pinholes on the mold-facing surface mean air was trapped against the silicone's walls and detail when you poured. Prevent them by brushing a thin first coat of resin into the mold's texture before the main pour, then tapping and gently pinching the mold walls so clinging bubbles break loose and rise.

How do you get bubbles out of resin without a torch?

Warm the resin bottles first so the mix is thin, let the mixed cup rest 5 to 20 minutes before pouring, then pop surface bubbles with a toothpick or a light mist of isopropyl alcohol. Exhaling warm air through a straw also works on small pieces. A torch is faster, but warm thin resin barely needs one.

Can you fix bubbles in resin after it has cured?

Yes. Fill scattered pinholes with a drop of UV resin or fresh epoxy and let it self-level. For a badly peppered face, wet-sand at 400, 600, then 1000 grit until evenly matte, wipe clean, and apply a thin flood coat — it cures back to full gloss. Deep voids get filled in layers first.

Should I pour resin hot or cold?

Pour resin warm, never cold. Stand the bottles in warm (not hot) water for 5 to 10 minutes so the resin flows like light syrup — thin resin releases bubbles easily, while cold thick resin traps them. Keep the room around 21 to 24°C too, since a cold workspace re-thickens the mix as you work.

Do I need a pressure pot for resin casting?

Not for most craft work. The warm-mix-rest-torch routine eliminates nearly all bubbles in coasters, trays, and jewelry. A pressure pot only becomes worth it for thick, glass-clear casts sold professionally, where it compresses any remaining bubbles to invisibility. Start without one; upgrade only if deep casts demand it.

Ready for a flawless pour? Browse high-detail, easy-release shapes in our resin mold collection and put the no-pinhole routine to work.