Acrylic aquarium crack repair: Weld-On 4, Weld-On 16, and the polish-out

Why acrylic fails the way it does

Acrylic (poly-methyl methacrylate, PMMA) is a thermoplastic, not a brittle ceramic. It does not snap on impact the way glass does. Instead it accumulates stress, and stress finds its way out through one of four failure modes. You need to identify which one you have before you choose a repair approach.

Crazing. A network of fine white hairlines that look frosted from certain angles. Crazing is the early stage of stress failure. It often appears at drill-through points, at the inside corners of overflow boxes, and along the top edge of a tank that has been sitting too long without a proper top brace. Crazing means the polymer chains have started to separate. Caught early, it is repairable. Caught late, the crazed zone progresses to a through-going crack and the panel is done.

Stress crack. A single crack that propagates from a stress riser - usually a drilled bulkhead hole that was not properly chamfered, or a corner where solvent pooled during original construction. Stress cracks travel slowly. You may have months between when the crack appears and when it reaches a free edge. Repair with Weld-On 16 and a stop-drill at the leading tip is feasible.

Impact spider. A radiating fracture from a single impact point. Common on tank lids where someone dropped a magnet cleaner, or on the front of a tank that took a hit from a vacuum. Impact spiders have through-thickness damage at the center; even with repair the optical clarity at the impact point will not return.

Seam separation. The solvent joint between two panels has let go. This is structurally identical to a silicone seam failure on a glass tank, but the repair chemistry is different. You will rebuild the seam using Weld-On 4 capillary action or Weld-On 16 if there is a visible gap.

Determining failure mode is a visual exam in raking light. Hold a flashlight nearly parallel to the panel and look across the surface. Crazing shows as a frosted patch. A stress crack shows as a single sharp line with no branching. An impact spider shows as a star pattern with a center point. A seam separation shows as a daylight gap at the joint line.

Weld-On 4 vs Weld-On 16 - which to use

The two solvents are sold side-by-side at Tap Plastics, US Plastics, and online via Amazon, and they look almost identical. They are not interchangeable. Choosing the wrong one is the most common acrylic-repair mistake.

For aquarium crack repair specifically, the right choice is almost always Weld-On 16 on the joint, optionally followed by Weld-On 4 wicked into any remaining hairline. Weld-On 4 alone cannot fill a visible gap; if you can see daylight through the crack, capillary solvent will not stay in place long enough to bond.

Acrifix 192 is a European alternative (UV-cure acrylic, water-clear after cure) used by some custom builders. It is excellent for visible repairs because the bond line is essentially invisible after cure, but it requires a UV lamp ($30-60) and the chemistry is fussier than Weld-On 16.

Materials and tools shopping list

• Weld-On 16 thickened cement ($18) - the primary structural repair material
• Weld-On 4 capillary solvent ($14) - finishing wicks and hairline closure
• Weld-On applicator needles ($8 for a 5-pack) - 25 gauge stainless steel, fitted to the cap of a small applicator bottle. Without these you cannot do precise capillary work.
• Plastic applicator bottles ($6 for a 3-pack) - the kind sold for hair dye. Methylene chloride will dissolve most squeeze bottles within hours, so transfer only what you will use in one session.
• 3M 6200 half-face respirator with 6001 organic vapor cartridges ($45 for the kit) - non-negotiable for indoor use
• Nitrile gloves, 8 mil thickness ($12 / box) - thin latex or vinyl gloves are dissolved by MC in seconds
• Drill with 1/8-inch and 1/4-inch bits - for stop-drilling crack tips
• Acrylic scraper or single-edge razor blade ($4) - for cleaning the bonding surfaces
• Novus 1 / Novus 2 / Novus 3 polishing system ($25 for the three-bottle kit) - the standard polish-out progression for the visible repair zone
• Micromesh sanding pads, 1500 / 2400 / 3600 / 4000 / 6000 / 8000 / 12000 ($20 for the assortment) - for wet-sanding before final polish
• Lint-free microfiber towels - cotton fiber gets pulled into the bond line and shows forever
• Spring clamps or band clamps - to hold the joint at 5 PSI minimum during set

Total materials cost for a typical repair: $130-170 if you do not already own respirator or polishing supplies. Most existing-supply tank owners spend $35-50 in fresh consumables per repair.

Step 1: Evacuate, drain, and dry completely

Solvent welding requires bone-dry bonding surfaces. Any moisture in the joint creates a cloudy white seam (the solvent picks up water and hazes during cure). Plan for at least 48 hours of drying time after the tank is fully drained.

1. Move livestock to temporary housing matched within 2 F of original temperature. See our livestock evacuation procedure for the full transport protocol.
2. Drain the tank completely. Substrate must come out for any crack repair on the bottom panel or any seam involving the bottom. For side-panel cracks above the crack line, you can leave damp substrate in place if you drop water to 6 inches below the crack.
3. Pull all equipment. Heaters, return pumps, powerheads, sump, plumbing. Any plastic part contaminated with MC during repair is permanently damaged.
4. Wipe interior with paper towels. Then leave the tank in a low-humidity space (40 percent RH or below) for 48 hours with a fan circulating air across the repair zone. In humid climates, set up a small dehumidifier nearby.
5. Final dry check: tape a piece of dry paper towel to the repair zone overnight. If it shows any dampness in the morning, dry another 24 hours.

Step 2: Stop-drill the crack tip (for stress cracks)

This step applies only to single-line stress cracks, not to crazing or impact spiders. The principle: a crack propagates from its sharpest point. Drilling a small round hole at the very leading edge of the crack converts that sharp point into a smooth radius, which drops the stress concentration at the tip by 80 percent or more. The Weld-On repair then has a much easier job because the underlying force driving the crack has been mitigated.

1. Locate the exact tip of the crack using a magnifier and raking light. Mark it with a fine permanent marker on the outside face of the panel.
2. Center-punch lightly with a spring-loaded punch or a tap from a finishing nail. This gives the drill bit a starting dimple.
3. Drill straight through with a 1/8-inch acrylic bit. Acrylic-specific bits have a zero-degree rake and a 60-90 degree point angle, which prevents grabbing and cracking around the hole. Hardware-store HSS bits at standard 118-degree point are NOT safe on acrylic - they will spider-crack the panel.
4. Run the drill at 500-800 RPM, very slow feed. Use water as coolant on a foam pad below the panel. Friction heat melts a halo around the hole and ruins the bond surface.
5. The hole becomes part of the repair. After bonding, the 1/8-inch hole gets filled with Weld-On 16 and polished flush.

Step 3: Surface prep

Acrylic solvents work by chemically dissolving the polymer surface molecules so they re-cross-link with their counterpart on the other side. Any contamination on the bonding surface - oil from skin, algae residue, polish wax, glass cleaner - blocks the dissolution and creates a weak bond.

1. Scrape any biofilm, calcium deposit, or coralline residue with an acrylic-safe scraper (Mr. Clean Magic Eraser works on stubborn deposits without scratching).
2. Wipe both bonding surfaces with isopropyl alcohol, 90 percent or higher. Avoid 70 percent isopropyl - it leaves a water film that haze-bonds.
3. Do NOT use Windex, glass cleaner, Goo Gone, or any solvent containing ammonia, acetone, or alcohol additives. These contaminate the bond surface in invisible ways.
4. Once cleaned, do not touch the bonding zone with bare hands. Fingerprint oil shows in the cured bond line as a yellow halo.
5. If the crack faces are jagged with internal lamination flaws, scrape lightly with a fresh single-edge razor at a 90-degree angle to bring the bonding faces back to flat. Do not over-scrape - the goal is removing high spots, not creating a new gap.

Step 4: Apply Weld-On 16 to the crack

1. Position the tank so the crack is horizontal and gravity holds the solvent in the joint. If the crack runs vertically you will have to either rotate the tank or work in 4-inch sections, letting each section set before moving up.
2. Cut the Weld-On 16 nozzle to a 1/16-inch opening. The thickened solvent should flow out as a continuous bead, not as drops.
3. Apply along the full length of the crack on one face only. The solvent will wick partway through the thickness by capillary action.
4. Apply gentle clamping pressure perpendicular to the crack. Spring clamps with cork-faced pads work well; aim for 5 PSI of clamp pressure (a 30-pound clamp force spread across a 6-square-inch zone). Too much pressure squeezes all the solvent out and starves the bond. Too little leaves gaps.
5. Watch for solvent bubbling at the crack edges - this is normal and indicates the dissolution is working. Tiny bubbles in the bond line are the polymer breathing. Big bubbles ( 1 mm or larger) indicate the joint is starved or that you used Weld-On 4 in a gap that needed 16.
6. Hold clamp pressure for 2-3 minutes minimum. Walk away for 30 minutes before any handling.
7. After 2 hours, the joint is dry-to-touch. After 24 hours it has reached approximately 60 percent of full strength. After 48 hours it is at working strength. Full molecular cross-link cure is 7 days.

Step 5: Finishing pass with Weld-On 4

After the Weld-On 16 has set for at least 4 hours, inspect for any remaining hairline at the bond line. Capillary action with Weld-On 4 will close most micro-gaps invisibly.

1. Fill an applicator bottle no more than half full with Weld-On 4. Methylene chloride evaporates fast - you will lose half the bottle in 20 minutes if you leave it open.
2. Press the 25-gauge needle tip against the panel right at the visible hairline. Capillary action draws solvent in.
3. Apply along the full length of any visible hairline. The solvent disappears into the bond line within seconds.
4. Let dry 30 minutes before any handling, 24 hours before refill testing.

Step 6: Polish out the repair zone

A solvent weld done correctly will be structurally sound but visually distinct: the bond line shows as a slightly different refractive index from the surrounding acrylic. Polishing brings the surface back to optical clarity in the repair zone.

1. Wet-sand the repair zone starting at Micromesh 1500. Work in straight strokes parallel to the longest dimension of the tank, not in circles. Keep the surface wet with a spray bottle.
2. Step up through 2400, 3600, 4000, 6000, 8000, 12000. Each grit should remove the scratch pattern from the previous one. Total time about 15 minutes per square foot.
3. Switch to Novus 3 (heavy cut polish) with a microfiber cloth. Apply with light pressure in straight strokes. Wipe off completely between coats.
4. Move to Novus 2 (mild cut). This is the workhorse polish - it removes the haze left by Novus 3 and produces a mirror finish on the bond line.
5. Finish with Novus 1 (clean and protect). This adds a thin anti-static coating that helps prevent dust accumulation during cure.
6. The repair zone will be slightly more reflective than the surrounding panel because it is freshly polished. After 30 days of normal aquarium service the panel surface refraction equalizes.

Step 7: Pressure test before livestock

Same protocol as any tank repair. Fill with dechlorinated fresh water only, mark the water line, leave for 48 hours minimum. Inspect the repair zone at 4, 12, 24, and 48 hours. Look for any weeping, any darkening at the bond line, any micro-bubbles forming under the surface.

On acrylic specifically, also check for "blooming" - a milky haze that develops in the bond line if any moisture was trapped during cure. Blooming cannot be polished out; if it appears, the repair must be redone.

When acrylic repair is not the right call

• Crazing that covers more than 4 square inches. The polymer chain damage extends invisibly into the surrounding material. Repair holds in the visible zone but the area around it fails within 12-18 months. Replace the panel.
• Impact spider on a viewing panel of a high-value reef tank. The optical distortion at the center point cannot be fully polished out. For a display tank, replace the panel.
• Multiple cracks on the same tank. The tank has accumulated stress across its lifetime. One repair may hold but another crack will appear within months. Retire and replace.
• Acrylic over 12 years old. UV exposure embrittles acrylic over time. Older Tenecor and SeaClear tanks crack easier and bond worse. Repair is a holding action, not a fix.

Acrylic vs glass repair cost comparison

For a typical hairline repair on a 75-gallon tank:

• Glass overlay repair: $30-60 in materials, $0 if you own basic tools
• Acrylic Weld-On repair: $130-170 in materials including respirator and polish kit, less if you already own polishing supplies
• Professional acrylic repair (Tenecor, custom acrylic shops): $200-450 plus shipping if the tank can be shipped
• Tank replacement (75g acrylic): $400-600 standard, $800-1200 for low-iron or rimless variants

The math usually favors repair on acrylic tanks over $300 retail. Below that price point, replacement is often more economical once you factor in time and materials.

FAQ

Need a replacement acrylic tank?

Tenecor, SeaClear, Pro Clear, and Clear-for-Life ship custom-spec acrylic tanks in 3-5 weeks. For overnight replacement when livestock is in jeopardy, glass alternatives ship same-day from most Fast Aquatics vendors.

Related guides

• Glass aquarium crack repair
• Silicone re-seam walkthrough
• Leak diagnosis flowchart
• Livestock evacuation procedure