What causes paint to crack in an alligator pattern on exterior siding in New Brunswick?

Alligatoring — that distinctive cracked, scaly pattern that looks like reptile skin — is almost always caused by paint layers that have lost their flexibility and can no longer expand and contract with the substrate underneath. In New Brunswick, our climate practically guarantees this will happen eventually if the wrong paint was used or if layers were built up improperly over the years.

The physics of alligatoring are straightforward: wood siding expands and contracts with every change in temperature and moisture level. NB experiences enormous swings — a July afternoon in Saint John might be 28°C and 80% humidity, while January nights can drop to -25°C with bone-dry cold. That is a dramatic range for a painted wood surface to endure. Paint that remains flexible — modern 100% acrylic latex — can stretch and compress with the movement without cracking. But paint that has hardened and become rigid over time will crack instead of flex, and the cracks propagate in that characteristic alligator pattern following the grain and stress lines of the wood.

In older NB homes, particularly those built before the mid-1990s when oil-based and alkyd exterior paints were the norm, alligatoring is extremely common. Oil-based paints cure hard and brittle, and they continue to harden with age. After 10-15 years, an oil-based exterior finish has lost most of its flexibility. If a second or third coat of paint was then applied on top — especially with a latex paint over an old oil-based coat without proper cleaning and priming — the incompatibility of the two layers causes the top coat to peel and crack as the bottom layer moves differently. The layer-cake effect of 40-60 years of repainting on old Fredericton or Moncton clapboard houses often results in a thick, rigid paint film that cracks dramatically.

Poor application practices also contribute. Paint applied too thick in a single coat, paint applied before the previous coat was fully cured, or paint applied in cool conditions that slowed proper curing — any of these can create internal stress in the film that leads to cracking as the paint ages.

The fix requires commitment to proper prep. Light surface alligatoring can sometimes be scraped, sanded, primed, and repainted. But severe or deep alligatoring — where the cracking goes through multiple layers down to bare wood — requires removing all the old paint, ideally back to bare wood. This can be done by hand scraping, chemical paint strippers, or heat guns (never heat guns on suspected lead paint surfaces). Once you are down to bare wood or a stable sound layer, prime with a quality exterior alkyd or acrylic primer and apply two coats of a premium 100% acrylic latex exterior paint.

A word of caution for pre-1978 NB homes: those many layers of old paint often contain lead, especially on trim and siding. Scraping or sanding alligatored surfaces without proper containment creates toxic lead dust. Have the paint tested before any aggressive removal work, and if lead is confirmed, hire a professional with lead-safe work practices. Never do aggressive paint removal on older NB homes without knowing what you are dealing with.

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