Here's the honest truth, mate: "paint and primer in one" is mostly clever marketing. What it really means is the paint is high-build enough to give decent coverage and adhesion on a surface that's already sound and previously painted. On that kind of job — a sound emulsioned wall going a similar or darker shade — it does the job and you can skip a dedicated primer.
Where it falls down is anything genuinely needing a primer's actual job: sealing, blocking, or adhering to a tricky substrate.
A few scenarios where you absolutely should NOT trust an all-in-one:
- Bare plaster. New plaster needs a proper mist coat (watered-down emulsion) or a dedicated plaster primer to seal the suction. An all-in-one topcoat will flash and patch.
- Stains, water marks, nicotine, knots. These need a stain-blocker. Reach for Zinsser BIN (shellac, blocks anything) or Cover Stain. No self-priming emulsion will stop a tannin stain bleeding through.
- Bare wood, MDF, gloss surfaces. You want a genuine adhesion primer or a degloss-and-prime routine. For cabinets, Bedec MSP or a Zinsser primer under your topcoat beats any "primer included" claim.
- Big colour change, especially over to a deep or bright shade. A grey or tinted undercoat does far more than a self-priming topcoat to cut your coats down.
For the everyday repaint, though, the quality trade emulsions from the good brands already perform like this without shouting about it. Little Greene Intelligent Matt, Dulux Diamond Matt, or Benjamin Moore Regal Select all build well and grip a sound surface — call it self-priming if you like. Going for something like Farrow & Ball Pointing or Cornforth White over an existing pale wall? You'll be sorted in two coats, no separate primer needed.
Practical rule: if the surface is sound and painted, the topcoat is your primer. If it's bare, stained, shiny, or dodgy — prime it properly. Don't let a label talk you out of doing the prep.