Yes, prime it. New bare wood is thirsty and porous, and if you go straight in with topcoat you'll get patchy absorption, raised grain, and a finish that never quite levels out. Priming seals the surface so your topcoat sits on top where it belongs rather than disappearing into the timber.
The bigger issue is knots. Softwood — your standard skirting, architrave, and pine — is full of resinous knots that will weep through paint and leave brown stains, especially under pale colours. No amount of topcoat fixes that once it's happened. The fix is a shellac-based knotting primer, and Zinsser BIN is the gold standard. Spot-prime the knots first, then prime the whole lot. For larger areas or where you want one product to do everything, Zinsser Cover Stain is a cracking oil-based primer that blocks knots and stains across the full surface.
Process that works every time:
1. Sand the bare wood with 120–150 grit, then dust off. 2. Spot-prime knots with Zinsser BIN. 3. Prime the full surface — Zinsser BIN or Cover Stain. 4. Light sand (240 grit), then two coats of your topcoat.
For the topcoat itself, Little Greene Intelligent Eggshell and Farrow & Ball Modern Eggshell are both excellent, hard-wearing water-based finishes for trim. If you want a traditional oil-rich feel, Dulux Heritage does a lovely eggshell too.
"But what about the all-in-one paints that say no primer needed?" Ignore them on bare softwood. They're fine over already-painted, sound surfaces — not raw timber with live knots. Manufacturers say "self-priming" to sell convenience; the knots don't read the tin.
If you're painting trim, Slipper Satin, Pointing, or a soft Ammonite are reliable off-whites that won't fight the wall colour. Prime properly underneath and they'll hold up for years.