Interior vs Exterior Paints: Differences & How to Choose

1) Core Differences

Aspect	Interior Paint	Exterior Paint
Resin/Binder	Hard, stain-resistant acrylics for scrubbability	Flexible, UV-stable acrylics to handle expansion/contraction
Additives	Mildew-resistant (light), low-odor/low-VOC, anti-stain	Heavy UV blockers, anti-mildew/anti-mould, anti-chalk, anti-salt (coastal lines)
Finish Options	Flat/Matte, Eggshell/Low-sheen, Satin, Semi-gloss	Low-sheen/Satin (walls), Semi-gloss/Gloss (trim, doors) for weather wash-off
Durability Focus	Washability, stain & mark resistance	UV, rain, wind, temperature swings, salt, dirt shedding
Flexibility	Moderate (stable indoor temps)	High (thermal movement, micro-crack bridging)
Odor/VOC	Low/Zero-VOC commonly available	May be higher (still many low-VOC options exist)
Primers	Stain-blocking, adhesion primers for plaster/gyprock	Masonry, multi-surface, tannin-blocking, anti-corrosion primers

2) Selection Criteria (Australia-savvy)

1. Location & Substrate

   • Interior gyprock/plaster: Interior acrylic + appropriate primer.

   • Exterior masonry/render: Exterior acrylic with masonry primer.

   • Timber (exterior): Tannin-blocking primer + flexible exterior topcoat.

   • Metal (exterior): Anti-corrosion primer + exterior enamel/acrylic.

2. Climate & Exposure

   • High UV (Perth/Adelaide/central): Exterior with strong UV inhibitors, mid-sheen for dirt shedding.

   • Coastal (Sydney/Newcastle/Gold Coast): Salt-tolerant systems; rinse periodically.

   • Humid (Brisbane/Cairns): Mould-resistant formulas; ensure ventilation.

   • Cooler zones (Canberra/Tasmania): Products that cure well at lower temps.

3. Sheen & Room Use (interior)

   • Ceilings: Flat/Matte (hide defects).

   • Living/Bedrooms: Low-sheen/Eggshell (balance look & cleanability).

   • Kitchens/Baths/Laundries: Satin/Semi-gloss, moisture & scrub resistant.

   • Trim/Doors: Semi-gloss/Gloss (harder, easier to wipe).

4. Health & Indoor Air

   • Prefer Low/Zero-VOC indoors; ventilate well.

   • Sensitive areas (nursery): certified low-VOC + mould resistance.

5. Maintenance & Colour Longevity

   • Exterior light colours chalk less visibly and reflect heat.

   • Dark exteriors look great but may heat more; pick UV-stable ranges.

   • Choose washable interior paints for high-traffic walls.

   
   

3) Quick Decision Flow

1. Where? Interior → go low/zero-VOC acrylic. Exterior → UV/mould/salt-resistant exterior acrylic.

2. What surface? Plaster/Timber/Metal/Masonry → match primer to substrate.

3. Exposure? High sun / coastal / humid / cold → pick specialised exterior line.

4. Finish? Practical cleanability vs desired look (low-sheen walls, gloss trims).

5. Colour & Maintenance? Lighter exteriors = cooler & cleaner look; interiors consider lighting.

   
   

4) Common Mistakes to Avoid

• Using interior paint outdoors (poor UV & weather resistance).

• Skipping substrate-specific primer (peeling, tannin stains, rust bleed).

• Choosing too flat a sheen on exterior walls in dirty/rain-exposed areas.

• Ignoring recoat & cure times —especially in humid/cold weather.

5) Minimal Starter Specs (Examples)

• Interior walls (living/bed) : Interior acrylic Low-sheen, 2 coats over sealer/primer.

• Wet areas (bath/laundry) : Moisture & mould-resistant interior acrylic, 2 coats.

• Exterior masonry : Exterior acrylic Satin, 2 coats over masonry primer.

• Exterior timber : Tannin-blocking primer + flexible exterior acrylic, 2 coats.

• Exterior metal : Rust converter (if needed) + anti-corrosion primer + exterior enamel/acrylic.