By True Shine Painters ·
The short version
- Exterior paint uses tougher, more flexible binders and additives to handle UV, moisture and movement.
- Interior paint focuses on scrub resistance, stain resistance and a smooth, even look.
- Use flat for ceilings, low sheen for most walls and semi-gloss for trims and wet areas.
- Do not use interior paint outside, and do not use exterior paint inside unless the label says so.
- Whatever you use, ventilate well and follow the label and data sheet.
What is actually different inside the tin?
Most house paint is a mix of four things: pigment for colour and coverage, a binder or resin that forms the film and holds everything together, fillers and additives, and a carrier such as water or solvent. The binder does the heavy lifting, and it is where interior and exterior paints differ most.
Exterior paints use binders and additives chosen for durability. They need to resist UV, which breaks down paint over time, stay flexible when timber and render expand and contract, resist water, and hold off mould and mildew. Many include additives to help with mould growth on the paint film.
Interior paints are built for a different set of problems. They aim to resist scuffs, marks and repeated cleaning, give an even finish under indoor lighting and cure to a smooth film. They do not need to handle sun, rain or big temperature swings.
Both types are commonly acrylic based in modern water-based paints, but the exact formulation varies between brands and ranges. Always check the manufacturer's data sheet for what a product is designed for.
How do UV, moisture and flexibility matter?
UV is the main enemy outside. Sunlight breaks down the binder and fades pigments, which is why cheap or unsuitable paint chalks and loses colour. Exterior paints are made with UV-stable resins and pigments to slow this down.
Moisture is the second. Exterior paint has to shed rain and dew, and cope with damp through winter, without blistering or letting water through to the surface. Interior paint in a dry room never faces that test.
Flexibility is the third, and it is often overlooked. A house is always moving a little. Weatherboards, render and fascias swell with moisture and heat and shrink again. A paint film that is too brittle will crack and then peel. Exterior products are formulated to stretch and recover, within limits. Rigid interior paint on an outside wall does not have that ability, so it cracks early.
Which sheen level suits which room?
Sheen is how much light the paint reflects. Higher sheen is generally harder wearing and easier to wipe clean, but it shows every surface flaw. Lower sheen hides imperfections and gives a softer look, but marks more easily.
The usual approach inside a Melbourne home follows this pattern, though your painter may suggest changes to suit the surface and lighting.
- Ceilings: flat, because it hides imperfections and does not glare under lights.
- Living areas and bedrooms: low sheen or matt walls, for a soft look that is still wipeable.
- Hallways and kids rooms: low sheen, or a tougher washable finish for scuff prone walls.
- Kitchens, bathrooms and laundries: low sheen to semi-gloss in a product rated for moisture and cleaning.
- Doors, skirting, architraves and window frames: semi-gloss or satin, for durability and easy wiping.
What sheen should I use outside?
Outside, most walls are painted in a low sheen or satin finish, and trims, fascias and doors in semi-gloss or gloss. A slightly higher sheen sheds water and dirt more easily, which helps in shaded and damp spots. Very flat finishes can hold dirt and mould more readily in some conditions, and manufacturers often suggest specific sheens for specific surfaces.
Sheen also affects how a wall looks in strong Australian light. A high gloss will show every ripple in weatherboards or render, so many people prefer a low sheen on large wall areas. Check the colour and the sheen in daylight, on the actual wall, before deciding.
Can I use interior paint outside, or exterior paint inside?
Short answer, no. Interior paint used outside is likely to chalk, fade, crack and peel faster than exterior paint, because it does not have the UV protection, flexibility or mould resistance. You can end up repainting in a fraction of the time and paying for it twice.
The reverse is less obvious but still not a good idea. Exterior paint inside may have a stronger smell, may contain additives not meant for enclosed living spaces, can stay tacky or soft for longer, and is not made for scrubbing and scuff resistance. Some manufacturers make products approved for both uses, and the label will say so. If it does not, do not assume.
Undercoats and primers follow the same rule. Use products designed for the surface and location, and check compatibility with the topcoat. A painter who knows the products will match the system to your surface, which is one reason to ask what is being used and where.
What about VOCs, smell and ventilation?
VOCs are volatile organic compounds, chemicals that evaporate from paint as it dries and contribute to that fresh paint smell. Modern water-based interior paints are available in low VOC and zero VOC ranges, and these are popular for bedrooms, kids rooms and homes with anyone sensitive to smells. Oil-based and solvent-based products generally have stronger fumes.
Low VOC does not mean no ventilation. Open windows, keep air moving and give the room time to air out after painting, even with low odour products. Follow the label and data sheet for ventilation advice, especially in small rooms, bathrooms and when using primers or sealers.
If anyone in your home has breathing issues or is pregnant, ask your painter about low odour options and about how the job will be staged, so they can stay out of the freshly painted rooms. Beyond following the product label, ask your doctor if you have a specific health concern.
If you would like help matching the right products and sheens to your house, True Shine Painters gives free quotes across Melbourne's west.