What actually changes the coat count
One coat only stretches far enough for a touch-up in the exact same colour on a wall that's already that shade, there's no colour change for a second coat to finish covering. Two coats is the standard for any real colour change, and it's what our paint calculator defaults to. A third coat, or a first coat of primer, only enters the picture for two specific jobs: bare plaster or new plasterboard that's never been painted, and a dramatic colour jump, dark over light or the reverse, where a single wall colour can bleed or shadow through two coats of the new one. Dulux's own preparation guide is explicit that an already-painted wall in good condition doesn't need an undercoat at all: “if the surface is already painted and in good condition, with no stains or mould, you shouldn't need an undercoat,” while bare walls get one full coat of sealer undercoat before any topcoat goes on.
What it actually costs
Same worked example as our paint calculator: a 4 m × 6 m room at the standard 2.4 m ceiling height, 48 m² of wall, budget-tier paint throughout.
| Option | What it includes | Cost |
|---|---|---|
| 1 coat, same-colour touch-up | 3 L, enough only if the wall's already this colour and in decent order | $21–30 |
| 2 coats, standard repaint | 6 L, the default for any real colour change on a wall that's already painted | $42–60 |
| 3 coats, no primer, bare plaster or a big colour jump | 9 L of topcoat only, three straight coats and no primer step at all | $63–90 |
| Primer + 2 coats, the other route to the same job | 4 L of primer/sealer/undercoat plus the standard 6 L of topcoat | $130–156 |
Paint coverage 16 m²/L per coat, primer/sealer/undercoat coverage 14 m²/L per coat (Dulux's own product figures, see sources). Litres rounded up to the nearest whole litre, the same way the calculator itself rounds.
Does a tinted primer actually save you money?
The advice repeated across most painting guides is that primer is the cheap insurance: swap one topcoat for one primer coat and you come out ahead, because primer is meant to cost less than paint. That holds in places where basic primer is priced like budget paint. It doesn't hold at Bunnings on the numbers above. A litre of Dulux 1Step Prep or Taubmans's 3-in-1 primer/sealer/undercoat runs $22–24, roughly two-and-a-half times what a litre of budget wall paint costs ($7–10). Do the sum for our worked room and the primer route, 4 L of primer plus the standard two topcoats, comes to $130–156, against $63–90 for three straight coats of the same budget paint with no primer at all. Run the same comparison in a premium brand and the gap narrows but doesn't flip: $171–243 for three premium coats versus $202–258 for primer plus two. On raw materials cost, skipping primer and adding a third coat is the cheaper way to cover a colour change, in both paint tiers.
What primer buys instead isn't a lower price, it's a coat count you can actually rely on. Bare, porous plasterboard needs a sealer coat regardless of price, or the topcoat goes on patchy and uneven. And for a genuinely dark-to-light repaint, a primer tinted close to the new colour at the paint desk (most stores, Bunnings included, will do this for the cost of the tin) is what actually caps the job at two topcoats. Skip primer on a strong dark base and three coats isn't a guarantee, it's a best case. If the old colour still shows through, the fourth coat you end up buying erases whatever the “cheaper” three-coat route saved, and then some.
Common questions
Do you need primer before repainting a wall?
No, not if the wall is already painted and in good condition, Dulux's own guidance is that a sound, unstained, mould-free painted surface doesn't need an undercoat. Bare plaster or new plasterboard is the exception: that gets one full coat of sealer undercoat before any topcoat, non-negotiably.
Does a tinted primer save money over just adding a third coat?
Not on Australian retail prices, no. A litre of primer/sealer/undercoat (Dulux 1Step Prep, Taubmans 3-in-1) runs $22–24, well above budget paint's $7–10/L, so primer plus two topcoats costs more in raw materials than three straight topcoats of the same paint, $130–156 against $63–90 for a 48 m² room in budget paint. Primer's value is a coat count you can rely on for bare plaster or a dark-to-light jump, not a lower price.
How much primer do you need before painting?
About 14 m² per litre per coat, slightly less than paint's 16 m²/L, per Dulux's own product coverage figures. A 48 m² room (our paint calculator's 4 m × 6 m worked example) needs 4 L of primer for one full coat, rounded up to the next whole litre.
Does the same 'check the real numbers before assuming which option costs less' pattern show up anywhere else on this site?
Yes, gravel driveway weed matting runs into a mirror version of it. Our weed membrane guide finds a $29 budget mat needing relaying every 2 to 4 years works out at $1,527–$5,905 over a decade once labour's counted, against a $44 commercial fabric installed once, the opposite verdict to primer here (the cautious-looking extra step costs more, not less) but the same underlying discipline: don't trust the sticker price, do the sum.
Sources
Primer/undercoat necessity and bare-wall preparation: Dulux, Guide to Painting Preparation. Primer coverage rate (14 m²/L): Dulux, 1Step Prep Water Based Primer, Sealer & Undercoat product page. Primer/undercoat retail pricing: Bunnings, Dulux 4L 1Step Prep Primer Sealer & Undercoat and Bunnings, Taubmans 4L White 3 in 1 Primer Undercoat & Sealer. Tinted-primer coverage benefit on dark-to-light repaints: Livingetc, What Is Tinted Primer?. Paint coverage rate, budget/premium $/L bands and the worked room: same Dulux/Bunnings sourcing as our paint calculator. All fetched July 2026.