How do I get a smooth finish on a painted door?

A door is the most looked-at painted surface in a house and the easiest to get visibly wrong, because it is flat, vertical and lit from the side.

Safety

Step by step

1. Decide whether to take it off.Painting a door flat on trestles removes runs entirely and gives the best finish. Painting it hanging is quicker and perfectly good with care. For a front door or a feature door, take it off.
2. Remove the handles and hardware.Not masked — removed. Ten minutes, and paint edges round masked hardware are visible from across a room.
3. Clean it before sanding.Sugar soap, especially round the handle and the bottom rail. Sanding a dirty door grinds the dirt in rather than removing it.
4. Sand the whole surface, not just the damage.180 to 240 grit over everything to take the sheen off. New paint needs a key across the entire door, not only where the old paint is chipped.
5. Deal with knots and bare wood.Knotting solution on any knot, then primer on any bare timber. Untreated knots bleed resin through the topcoat months later, and it comes back every time.
6. Fill, sand and spot-prime.Two thin fills rather than one deep one, sanded flush, then primed. Unprimed filler shows as a dull patch through the finish.
7. Wipe the dust off properly.Vacuum, then a tack cloth or a barely damp cloth. Dust in the film is the most common cause of a gritty finish and it is entirely avoidable.
8. Learn the order for a panelled door.Mouldings and panel recesses first, then the panels, then the vertical centre pieces, then the horizontal rails, then the outer stiles. That sequence keeps a wet edge and stops overlaps showing.
9. Do a flush door in sections quickly.Divide it into thirds, roller each section on and lay it off with a brush immediately, working into the previous section while it is still wet.
10. Load the brush properly.A third of the bristle depth, tapped rather than scraped. A dry brush drags and leaves marks; an overloaded one runs.
11. Lay off in one direction, then stop.Light strokes with the grain, then leave it alone. Going back over paint that has begun to set is what creates most brush marks.
12. Sand lightly between coats.A gentle pass with fine paper and a wipe down between each coat. This step, more than the brushing, is what produces a smooth finish.
13. Do the edges deliberately.The hinge edge and the latch edge get handled and knocked. The top and bottom edges should be sealed too — unsealed end grain absorbs moisture and causes sticking.
14. Let it dry before closing it.Closing a door onto a frame before the paint has hardened sticks them together and pulls the finish off. Leave it open overnight.

Tips

• Mouldings, panels, centre verticals, rails, outer stiles. Follow that order and lap marks largely disappear.
• Lay off once and stop. Reworking half-set paint causes more brush marks than a cheap brush does.
• Light sand between coats. It is the single biggest contributor to a smooth finish.

Common mistakes

• Going back over paint that has started to set — The brush drags through a skinning surface and leaves permanent marks. Lay it off once while wet and leave it.
• Not treating knots — Resin bleeds through the topcoat as brown patches weeks or months later, and it recurs until the knot is sealed with knotting solution.

If it doesn't work