Why old paint cracks like crocodile skin

Paint that cracks into a pattern like crocodile skin has become too hard and brittle to move with the surface underneath it, so it splits into scales. It cannot be fixed by painting over it: the cracked layers have to be removed back to a sound base, and the new system built up from there.

The trade names for it are alligatoring, crocodiling or mudcracking. It is a different defect from blistering or peeling, and it needs a different answer.

What causes alligatoring?

Resene's technical list of paint failures gives two usual causes: solventborne paint applied over soft waterborne paint, and an old solventborne paint simply becoming too brittle over time to cope. Dulux's problem solver adds a third: a topcoat applied before the undercoat underneath it had dried.

All three come down to the same mismatch. The top layer is harder and less flexible than what it sits on, or than the timber it is attached to. Timber swells when wet and shrinks when dry, and it expands in the sun. A flexible film moves with it. A rigid film cannot, so it cracks, and the cracks follow the path of least resistance into that familiar grid of scales.

Cause	What it looks like	Where you usually see it
Old oil-based paint gone brittle with age	Fine grid of cracks through the whole film, often down to bare timber	Sunny elevations, sills, fascias and doors on older houses
Hard coat over a softer coat	Cracks through the top layer while a different-coloured layer shows in the gaps	Where enamel was put over acrylic, or many layers have built up over the decades
Recoated before the lower coat dried	Cracking in relatively recent paintwork rather than decades-old layers	Recent work, especially thick coats or cold, damp drying conditions

Why does it show up on old houses so often?

Because a typical villa or bungalow is carrying decades of paint. Many of those layers were oil-based, some are acrylic, and they were put on by different people with different preparation. Every repaint adds thickness, and a thick stack of mixed coatings is exactly the recipe for one hard layer sitting over a softer one.

Sun makes it worse. Oil-based paint keeps hardening as it ages, and the elevations that take the most sun age fastest. That is why crocodiled paint is often worst on a north-facing wall or a window sill, while the shaded side of the same house still looks intact.

Can you just paint over it?

No. This is the question that matters, because painting over alligatored paint is where money gets wasted.

New paint over a crocodiled surface will look good for a while. It fills the cracks visually and gives a uniform colour. But it is now bonded to a layer that is already broken into loose scales and has lost its grip on what lies beneath. As the old film keeps moving and lifting, it takes the new coat with it. The cracks telegraph back through, usually within a season or two, and then you have a failed new coat on top of a failed old one, which is more material to remove.

Resene's advice is that the best rectification is to totally remove the paint and start again. Dulux's remedy is the same: remove the old paint, then prime and repaint.

The only partial exception is fine surface checking that sits in the top coat only, where the layers beneath are sound and firmly bonded. Test it. Press a strip of strong tape firmly over the cracked area and pull it off sharply. If flakes come away on the tape, the cracking goes deeper than cosmetic, and it has to come off.

What does fixing it involve?

It is a strip-back job, which is why it costs more than a standard repaint. In rough order:

1. Test for lead first. Homes painted before the 1980s may contain lead-based paint, and the old oil layers that crocodile are exactly where it is likely to be. Have it tested before sanding or stripping. If it is positive, sheet the ground, avoid dry sanding and keep children and pets away from the work area. There is more in our guide to lead paint removal.
2. Remove the failed paint. Resene's exterior timber guide notes that a hot air gun is very slow, which is why most professional painters use a mechanical stripper or a carbide scraper on unstable paint, with some risk of gouging the timber. Chemical strippers are another option on detailed joinery.
3. Sand and assess the timber. Once the paint is off, you can see whether the timber has greyed, cracked or rotted underneath. Rot is repaired or replaced before anything else.
4. Prime the bare timber the same day. The same guide warns that night dew creeps under freshly sanded paint edges and makes them lift, so stripping is planned in sections that can be primed before evening.
5. Fill, sand and build the new system. Undercoat where the product calls for it, then two topcoats, using one compatible system from primer to finish so the problem is not rebuilt.

On a house where only one or two elevations are affected, it is common to strip those sides and give the sound walls a standard wash, sand and repaint. That split is worth asking about, because it is often the difference between an affordable quote and an alarming one.

How is it different from blistering or peeling?

Blistering is paint lifting in bubbles, usually from moisture or heat under the film. Peeling is paint losing adhesion and coming away in sheets or strips. Alligatoring is the film itself cracking into a pattern while still partly attached. All three can end in flaking paint, but the causes are different, and so is the fix. Our article on blistering vs peeling paint covers the other two.

How do you stop it coming back?

• Do not put a hard solventborne enamel over a soft waterborne coat.
• Let each coat dry properly before the next goes on, and allow longer in cold or damp weather. Check the recoat time on the tin.
• Once a house has been stripped, keep to a flexible exterior system and stick with it at each repaint.
• Repaint before the film is worn through, so you are washing and recoating rather than stripping again.

Is it ever fine to leave it?

On a garden shed or a fence nobody looks at, crocodiled paint is an eyesore rather than an emergency. On weatherboards and joinery it is different. Every crack is a path for water into the timber, and that is where rot starts. If you are planning a repaint anyway, factor the strip-back into the budget rather than hoping a thick coat will cover it. Our exterior painting quotes itemise stripping separately so you can see where the money goes.

If you want someone to look at cracked paint and tell you whether it needs stripping or just a repaint, get in touch.