Short answer: Stop the crack first: drill a small hole, about 1/16 to 1/8 in., right at the tip (or each tip) of the crack so the stress spreads around a round hole instead of a sharp point. Then, with the crack clean and dry, wick in a water-thin solvent cement such as Weld-On 3, Weld-On 4, or SCIGRIP 4 with a needle applicator, and let it cure 24 hours. This stops the crack and makes it much less visible, but it won't restore full strength. Replace panels that hold water, weight, or people.
Key facts
- Stop hole: a small drilled hole at the crack tip, roughly 1/16 to 1/8 in., all the way through.
- Cement: water-thin solvent cement (Weld-On 3, Weld-On 4, SCIGRIP 4) wicks into tight cracks by capillary action, about 1/4 in. per application per IPS.
- Cure: Weld-On 3 reaches handling strength in minutes and 65 to 80% of ultimate strength in 24 hours.
- Limits: thin cement doesn't fill gaps. Open cracks need a thickened cement (such as Weld-On 16) or a patch.
- Replace instead for aquariums, shelves under load, guards, and windows exposed to impact.
Why do cracks in acrylic keep growing?
A crack ends in a sharp point, and a sharp point concentrates stress many times over. Every flex, temperature swing, or tightened screw pushes it a little further. Plaskolite's fabrication guide notes that notches, holes, and grooves all multiply stress, and the tip of a crack is the worst notch of all. Drilling a round hole at the tip replaces the sharp point with a smooth curve, which spreads the stress and usually stops the crack.
How to stop and seal a crack, step by step
- Find the true end of the crack. Look at it at a low angle with a light behind it; cracks often run further than they look. Mark just beyond the tip.
- Drill the stop hole. Use a sharp bit ground for plastics (or a standard bit with the cutting edge dulled flat), low speed, light pressure, and the sheet backed with scrap wood. A 1/16 to 1/8 in. hole is enough on most sheet. Drill each end if the crack has two. See how to drill acrylic without cracking.
- Clean the crack. Blow out dust with compressed air. Don't use alcohol or glass cleaner in the crack; stressed acrylic crazes easily.
- Wick in cement. Lay the sheet flat, crack facing up. Touch the tip of a needle applicator filled with water-thin cement to the crack and let capillary action draw it in. IPS says Weld-On 3 travels about 1/4 in. into a tight joint per application, so work along the crack in short steps.
- Close the crack gently if it has opened, with light clamping or tape across it, without bending the sheet.
- Cure. Leave it flat and undisturbed for 24 hours. Strength keeps rising for days.
- Finish. Fill the stop hole with a short piece of acrylic rod and a drop of cement if you want, then trim it flush and polish.
Safety: solvent cements give off strong vapors. Use them with good ventilation and keep them away from flames; read the product's safety data sheet.
Which glue is best for a cracked acrylic sheet?
| Product | Type | Use for |
|---|---|---|
| Weld-On 3 | Water-thin, very fast | Tight hairline cracks; can blush white in humid air |
| Weld-On 4, SCIGRIP 4 | Water-thin, slower | Tight cracks; more open time and less blushing |
| Weld-On 16, SCIGRIP 16 | Thickened, gap-filling | Cracks that have opened slightly, or chipped areas |
| Clear two-part epoxy | Adhesive | Last resort for wider gaps; won't match acrylic's clarity |
Avoid cyanoacrylate (super glue) on clear parts: its vapors frost the surrounding acrylic white. Avoid silicone: it doesn't bond to acrylic structurally and it contaminates the crack so cement won't work later. See does silicone stick to acrylic and how to glue acrylic.
Will the crack disappear?
Usually not completely. Cement fills the crack and removes most of the reflection that makes it visible, so a hairline crack can become a faint line. Cracks with dirt in them, branching cracks, and cracks that opened before you bonded them stay more visible. Crazing (many tiny surface cracks) can't be fixed with cement at all; it has to be sanded and polished out. See what is crazing in acrylic.
When should I replace cracked acrylic instead?
- Aquariums, sumps, and anything holding water. A repaired crack is a weak line under constant load. Drain it and replace the panel or the tank.
- Shelves, steps, and anything carrying weight.
- Guards, windows, and doors that may be hit.
- Cracks longer than a few inches, branching cracks, or cracks starting from screw holes, which will just crack again unless the mounting changes.
For a cracked panel, tabletop cover, cabinet insert, or window, we cut a replacement to size with polished edges and drilled holes. Measure it with how to measure for acrylic cut to size, then price it in the instant quote.
How do I stop acrylic cracking again?
- Oversize screw holes and don't overtighten; use washers.
- Round inside corners and keep holes away from edges.
- Allow room for thermal expansion in frames.
- Keep solvents and alcohol cleaners off stressed edges, and anneal parts before cementing. See how to anneal acrylic.
- Use polycarbonate where impacts are expected; see acrylic vs polycarbonate.
Questions
Can I melt a crack in acrylic closed with a heat gun?
No. Heat won't fuse the crack faces, and it adds stress and distortion around the area. Use a stop hole and solvent cement.
Can I fix a cracked acrylic aquarium?
Not reliably at home. A stop hole and cement may slow a small crack in a non-structural area, but a crack in a wall or seam of a filled tank is a failure risk. Drain it and have it professionally assessed or replaced.
Why did my acrylic crack on its own?
Usually from built-in stress plus a trigger: a tight screw, a solvent cleaner, cold weather, or heat cycling. Laser-cut and flame-polished edges are common starting points; see why laser-cut acrylic cracks later.