Owner, Procover Painting · 14+ years painting · NSW Licence #392662C · 9 min read
Key takeaways
- All three are prepcoats. You apply them before the topcoat, and each one solves a different problem.
- Bare, stained or glossy surfaces need a primer. Very porous surfaces also need a sealer, or a primer-sealer.
- Undercoat is for smoothing and hiding. It matters most when you change from a dark colour to a light one.
- A sealer won’t fix peeling paint. Scrape and sand off loose paint first.
Quick answer: what is the difference?
Primer, sealer and undercoat are all “prepcoats”. You put them on before the topcoat to get a better result.
- Primer is the first coat on a surface. It helps paint stick and can block stains.
- Sealer is a type of primer that stops the topcoat soaking into the surface.
- Undercoat goes on after the primer. It fills minor imperfections and gives an even, solid base.
The names overlap, and some tins are a primer, sealer and undercoat in one. Always check the label for what it can do.
The three products explained
Primer
Gives paint something to grip on bare, new or glossy surfaces. Depending on the type, it can also block stains and tannins, resist rust, and add a moisture barrier.
Use it when: the surface is bare, stained or shiny, or you have repaired it.
Sealer
A type of primer used with, or instead of, a regular primer. It stops very porous surfaces soaking up the topcoat, so you get even colour and sheen. Some also add water resistance.
Use it when: the surface is porous, such as new plasterboard, bare masonry or filler patches.
Undercoat
Goes on over the primer, or over stable old paint. It fills minor imperfections and hides what’s underneath. It also helps when you go from a dark colour to a pale one.
Use it when: you want a flawless finish or you’re changing colour a lot.
Which one do you need?
Start with the surface you are painting, then match it to the problem.
| Your situation | What to use | Why |
|---|---|---|
| New or bare surface | Primer. Add a sealer, or use a primer-sealer, if it is very porous. | Gives the topcoat something to grip and stops uneven soak-in. |
| Old paint in good condition | Usually no primer. Spot-prime bare patches. | The old paint already gives the topcoat a stable surface. |
| Dark colour to a light colour | Undercoat, then topcoat. | Hides the old colour so you need fewer topcoats. |
| Stains (water, smoke, tannin) | Stain-blocking primer or sealer. | Stops stains bleeding through the topcoat. Fix the cause of water stains first. |
| Glossy surface | Scuff sand, clean, then a bonding primer. | Paint can’t grip smooth gloss on its own. |
| Peeling or flaking paint | Scrape and sand off loose paint, then prime bare patches. | New paint can’t hold on a surface that is already letting go. |
Can one tin do all three?
Yes, sometimes. Products sold as a “primer, sealer and undercoat” do all three jobs in one tin. Examples include Dulux 1Step (water-based and oil-based versions), Taubmans 3-in-1 and Wattyl Prep Acrylic Sealer Undercoat. They save time on general jobs. For hard jobs, such as heavy stains, glossy surfaces or bare metal, a specialist primer usually does better.
Surface-by-surface guide
Use this as a starting point. Surfaces and products vary, so always check the tin.
| Surface | Primer or sealer | Undercoat | Watch out for |
|---|---|---|---|
| New plasterboard | Acrylic primer-sealer for plasterboard. It evens out how the paper and joints soak up paint. | Usually not needed with a good primer-sealer. | Sand joints and dust off before priming. |
| Bare timber | A primer made for timber. Use a stain-blocking primer or shellac-based sealer over knots and tannin-rich timber. | Often used before gloss or semi-gloss topcoats. | Sand first. Fill nail holes after priming. |
| MDF | A primer suited to MDF. Seal the cut edges, as they soak up paint. | Optional, for a smoother finish. | Keep MDF dry. Water makes it swell. |
| Bare brick and concrete | A masonry primer or sealer made for masonry. | Optional on rough surfaces. | Don’t paint over damp walls or white salt deposits (efflorescence). |
| Cement render | A masonry or alkali-resistant primer or sealer suited to render. | Optional. | Let new render cure fully. Follow the render maker’s advice. |
| Bare steel | A rust-inhibiting metal primer. | Use a metal undercoat if the paint system calls for one. | Remove loose rust first. |
| Galvanised steel | Clean, lightly abrade, then use a primer made for galvanised surfaces (often an etch primer). | As directed by the paint system. | Ordinary primers can fail to grip. See our Colorbond painting page. |
| Glossy surfaces (laminate, tiles) | Scuff sand, clean, then a bonding primer. | Not usually needed. | Check the primer says it suits that surface. |
| Stained surfaces | A stain-blocking primer or sealer. | Usually not needed after a stain-blocking coat. | Fix the cause of water stains, or they will come back. |
Types of primer
Acrylic (water-based)
Suits most interior and exterior surfaces. Dries quickly and cleans up with water.
Oil-based
Grips tricky surfaces like timber and metal and blocks stains and odours. Dries slowly and cleans up with mineral turps. Dulux 1Step Prep Oil Based can sit under water-based or oil-based topcoats.
Stain-blocking
For water marks, smoke, nicotine and tannins in timber. It forms a barrier so stains don’t bleed through the topcoat.
Shellac-based
A fast-drying sealer made from lac resin. Very good at blocking stains, smells and tannins. Strong smell, so ventilate well. Clean up with methylated spirits.
Bonding
Made to grip glossy, hard-to-paint surfaces like laminate and tiles. Scuff sand and clean first.
Masonry
Made for brick, concrete and render. Some are alkali-resistant or anti-efflorescent. Check the tin.
Sanding old paint in a home built before 1970?
Treat it as lead paint until you know otherwise. Never dry sand it. Read the lead paint section in our oil vs water-based paint guide and the NSW LeadSmart advice before you scrape or sand.
Common priming mistakes
Skipping primer on bare patches
Filler, repairs and bare plaster soak up paint differently. They show through as dull or patchy spots, even after two topcoats.
Priming over dirt and grease
Primer can only grip a clean surface. Wash with sugar soap, rinse and let it dry first.
Using a sealer on peeling paint
Sealer won’t glue loose paint down. Scrape and sand it off, prime the bare patches, then repaint.
Painting over damp or salty masonry
White crusty salts (efflorescence) come from moisture moving through the wall. Brush them off, fix the damp, let it dry, then use a masonry primer.
Mixing incompatible products
Check that the primer suits your topcoat and the surface. Also check the recoat window. For example, Dulux 1Step Prep says to topcoat exterior work within a week.
Skipping the undercoat on a big colour change
Going from dark to light without an undercoat usually means extra topcoats and a patchy result.
Frequently asked questions
Primer is the first coat. It helps paint stick and can block stains. Sealer is a type of primer that stops a porous surface soaking up the topcoat. Undercoat goes on after the primer to give a smooth, even base and to hide colour underneath.
Usually not, if the old paint is in good condition and clean. Spot-prime any bare patches, repairs or stains. If the paint is peeling, scrape and sand it first.
Often, yes. Products sold as a primer, sealer and undercoat do all three jobs in one tin, and they suit many general jobs. For heavy stains, glossy surfaces or bare metal, use a specialist primer.
Going from dark to light usually benefits from an undercoat, because it hides the old colour. Going from light to dark often needs less, but still check the tin.
Follow the recoat time on the tin. For example, Dulux lists 1Step Prep Oil Based as touch dry in 30 minutes and recoatable after 1 hour. It says to apply the topcoat within a week for exterior work. Cold or humid weather slows drying.
Not by itself. Peeling paint needs the loose paint scraped and sanded off first. A sealer helps the new paint go on evenly over the repaired surface, but it won’t hold failing paint in place.
Not sure what your walls need?
Send us a few photos. We’ll tell you what we’d use, what prep it needs, and give you a clear written quote. We service Newcastle, Maitland, Port Stephens, Lake Macquarie and the Hunter Valley.
Sources and further reading
- Inspirations Paint, What is the difference between sealer, undercoat and primer?
- Screwfix, Difference between primer and undercoat (UK retailer guide).
- Dulux, 1Step Oil-Based Primer, Sealer & Undercoat (surfaces, topcoats, drying and temperature limits).
- Bunnings Trade, Taubmans 3-in-1 Interior/Exterior Primer, Undercoat & Sealer.
- Bunnings Workshop, How to paint galvanised steel posts (clean, abrade, galvanised primer).
- Rockcote, Removal of efflorescence from masonry before painting.
- NSW Government, LeadSmart: renovating and maintenance.
Last reviewed 19 October 2026. Product details were checked against the pages listed above. Always read the label on your paint before you start.
