Preparation starts with diagnosis, not sanding
The first question is not “Which filler should I use?” It is “What is wrong with this surface, if anything?” A sound painted wall may only need cleaning, small local repairs and a light key where appropriate. A wall with flaking coating, repeated cracks or active moisture needs a completely different response.
That judgement matters because both under-preparation and pointless over-preparation waste money. The goal is a stable, suitably smooth and compatible base — not performing every possible process on every surface.
Cleaning and contamination
Paint adheres to the surface it touches. Grease, nicotine, soot, hand marks, dust and biological contamination can interfere with adhesion or bleed through later.
Kitchens, fireplaces, exterior walls and older occupied properties often need more than a dry dust-off. The treatment should suit the contaminant and substrate; aggressive cleaning that damages the surface is not an improvement.
Loose, flaking and poorly adhered paint
New coating can only be as sound as the layer underneath it. If old paint is loose, brittle or poorly adhered, putting premium paint over it does not strengthen the base. Loose material needs to be removed back to sound edges, then the exposed substrate assessed.
On an exterior, this can reveal far more failure than was visible during the initial survey. That is why some preparation-heavy jobs look dramatically worse halfway through.
Cracks, dents, holes and filler
Small dents, screw holes and stable local cracks can usually be repaired as normal decorating work. The repair needs to dry, be sanded flush and be primed where the filler or substrate requires it.
Repeatedly opening cracks are different. If movement continues, more filler may only postpone the crack reappearing. Large areas of blown plaster, loose render or obvious structural movement need investigation rather than cosmetic filling.
Sanding: enough to create the right surface
Sanding can flatten filler, remove nibs, feather paint edges and create a suitable key on glossy surfaces. It should have a purpose. Heavy sanding of a sound matt wall achieves little beyond dust and labour.
On older painted surfaces, particularly where historic lead paint may be present, uncontrolled sanding can also be inappropriate. The age and condition of the coating matter when deciding the preparation method.
Stains: fix the source, then stop the stain coming through
Water marks, nicotine, soot, tannin and rust can migrate through ordinary emulsion. A suitable stain-blocking or isolating product may be needed after the source has been understood.
The crucial distinction is whether the stain is historic and dry or whether the problem is still active. A stain blocker can isolate a dry mark. It cannot repair an ongoing roof leak.
Primers, sealers and stabilisers
Primers are used for reasons, not as an automatic badge of quality. Bare timber, metal, porous repairs, difficult substrates, stains and certain exterior conditions may need a dedicated primer or sealer.
The right product depends on the problem and finish system. A universal “one primer for everything” approach is rarely as useful as understanding why the primer is needed.
Caulk and flexible joints
Fine gaps between trim and walls can often be finished with suitable decorator's caulk. It can improve a junction and accommodate small normal movement. It should not be used to disguise wide structural cracks or replace missing material.
Over-caulking can also soften crisp architectural lines and create rounded edges where the original detail should remain visible.
Preparing timber and woodwork
Woodwork preparation may include cleaning, deglossing, sanding damaged areas, filling local defects, dealing with knots or stains and priming exposed timber. Detailed doors, mouldings and windows need care so preparation does not destroy the profile it is meant to improve.
Rotten timber is a different problem. Small local repairs may be possible, but extensive decay should not be hidden with filler and paint.
Knowing when not to paint
Sometimes the correct decorating decision is to stop. Active moisture, rotten timber, unstable plaster, large hollow render areas or moving cracks need the underlying issue addressed first.
A decorator can often recognise warning signs, but they should not pretend paint is a building repair. The same applies to exterior coating failure; see the guide to common exterior paint problems.
What counts as over-preparation?
More preparation is not automatically better. Completely stripping a sound coating, sanding every wall heavily or priming every surface regardless of condition can add cost without improving the result.
The best preparation is proportionate: enough to create a sound, compatible base and resolve defects that will affect the finish. That is one reason a detailed survey matters more than promising a fixed list of processes before the surface has been seen.
Preparation is where judgement sits.
The visible work may be filling and sanding, but the important decision is knowing which defects matter, which products solve the right problem and when paint should wait until another issue is repaired.
This is general decorating guidance. Older coatings, moisture, significant cracking and specialist substrates can require testing or another trade before a painting specification is chosen.
