Alligatoring

Alligatoring is a coating defect in which the film develops a network of deep, wide cracks forming irregular rectangular or polygonal patches, like the scales on an alligator’s back. The cracks often extend through one or more coats and may reach the substrate. Alligatoring is most often seen on older paint on wood, metal and asphalt-based roofs, and it signals that the coating has lost the flexibility to accommodate stress. It is one of a family of cracking defects that includes mud cracking and checking.

Appearance

• Large, roughly rectangular islands of coating separated by cracks.
• Cracks are wider and deeper than fine checking.
• Islands may curl at the edges as the film shrinks.
• Underlying coats or substrate may be visible in the cracks.
• On asphalt and bituminous surfaces, the pattern is often accompanied by surface oxidation and loss of oils.

Causes

Hard coat over a soft or flexible coat

The most common cause. When a rigid topcoat is applied over a soft, slow-drying or more flexible undercoat, the two layers move differently. As the undercoat continues to shrink or move, the hard topcoat cracks. Examples include:

• Applying an enamel over a primer that is not fully dry.
• Applying a hard coating over an oil-based or bituminous layer.
• Recoating over a softer, incompatible product (see coating existing coatings).

Aging and embrittlement

Oil-based and alkyd coatings continue to oxidize and crosslink for years (see alkyd coatings). They gradually become hard and brittle. Temperature cycling then causes cracking, which deepens into alligatoring.

Excessive film thickness

Many layers of paint built up over decades, or a single very heavy coat, have less tolerance for movement. Thick films develop larger internal stresses as they cure and age.

Thermal cycling and substrate movement

Substrates that expand and contract, such as wood with moisture changes or metal roofs in sun, put repeated strain on coatings. Brittle coatings eventually crack.

Bituminous substrates

Coal tar, asphalt and bitumen release oils and soften in heat. Coatings applied over them can alligator as the base moves and bleeds (see bleeding and staining). Asphalt itself alligators with age as it oxidizes (see coating asphalt).

How alligatoring differs from other cracking

Diagnosis

1. Visual inspection. Note the pattern, crack depth and which layers are affected.
2. Cross-section examination. Cutting through the film with a knife or using a microscope shows how many coats are involved and where the crack starts.
3. Adhesion testing. Check whether the remaining coating is still bonded (see adhesion testing).
4. Coating identification. Solvent rub tests or laboratory analysis can identify incompatible layers (see FTIR analysis of coatings).
5. Hazard testing. Old paint may contain lead; test before disturbing it (see lead paint hazards).

Repair

Alligatored coatings generally cannot be repaired by painting over them, because the cracks will reflect through new coats and the old film remains weak.

1. Remove the defective coating. Methods include scraping, power tool cleaning, chemical paint stripping and abrasive blasting, chosen to suit the substrate and any hazards.
2. Prepare the substrate according to the new system’s requirements.
3. Select a compatible system. Choose a primer and topcoat designed to work together and suited to the substrate’s movement.
4. Apply at specified thickness and observe drying and recoat windows.

For asphalt surfaces, cracks are usually filled and the surface sealed or overlaid with a compatible system.

Prevention

• Allow each coat to dry fully before recoating.
• Do not apply hard, rigid coatings over soft or bituminous layers without a compatible barrier.
• Use flexible coatings on substrates that move.
• Avoid excessive film builds; follow wet film thickness measurement during application.
• Strip and restart when paint layers become too thick or brittle.

In practice

In practice, the decision on repair depends on how deep the cracking goes. A light surface pattern on a sound, well-bonded system can sometimes be sanded and recoated with a compatible flexible product. Deep alligatoring that reaches the substrate, or that sits over a soft or bleeding layer, usually means removal to a sound base. Testing adhesion and identifying each layer, for example with a cross-section or a small solvent test, helps avoid repeating the failure.

Key terms

• Checking. Fine, shallow cracks in the top layer of a coating.
• Crazing. A network of hairline cracks, often irregular.
• Cracking. Breaks that reach through the coating to the layer below.
• Hard over soft. Applying a hard coating over a softer, more flexible coat, a classic cause of alligatoring.
• Bitumen bleed. Migration of oils from asphalt or coal tar that can disturb topcoats.

Frequently asked questions

Why is it called alligatoring?

The pattern of cracks resembles the scaly skin of an alligator.

Can I sand and paint over alligatoring?

Sanding may smooth the surface but does not restore the weakened film. Cracks usually reappear. Removal is the reliable fix.

Is alligatoring the same as crazing?

Crazing is a fine network of hairline cracks, usually in clear or hard finishes. Alligatoring is coarser and deeper.

Related entries

• Mud cracking and checking
• Coating failure investigation
• Coating existing coatings
• Coating wood
• Telegraphing (new)
• Adhesive vs cohesive failure (new)