Interior vs. exterior paint: what actually changes

Every paint is a mix of pigment, binder, solvent and additives. Pigment gives colour, binder holds it to the surface, solvent carries it while wet, and additives handle everything else. Interior and exterior products differ almost entirely in that last category — and in how flexible the binder is.

Exterior paint is built to move

Siding expands in summer heat and contracts on a cold night. Exterior binders are formulated to flex with that movement instead of cracking. They also carry UV blockers to stop the sun breaking down the pigment, and mildewcides to resist growth in damp shade.

That flexibility is why exterior paint stays slightly softer once cured. Indoors, that softness is a liability: it holds fingerprints, picks up dirt, and never reaches the hard, scrubbable surface an interior wall needs.

Interior paint is built to be lived with

Interior products optimise for a different set of problems — low odour, low VOC content, minimal spatter, and a finish that survives being wiped down. They cure harder and hold a more consistent sheen across a large flat wall.

• Interior — harder cure, scrubbable, low-VOC, stable sheen.
• Exterior — flexible film, UV-resistant, mildew-resistant, water-shedding.
• Both — need clean, dry, properly primed substrate to bond.

Where the line genuinely blurs

A handful of spaces sit between the two: garages, covered porches, sunrooms and laundries all see temperature swings and moisture without full weather exposure. Hybrid interior/exterior products exist for exactly these, and they are the right call — not a compromise.

If you are unsure which category your project falls into, that is a normal question and worth asking before the first coat rather than after. We cover it during every free estimate.