Most filler failures are selection failures. The product was applied correctly, but it was the wrong product for the depth, the substrate or the finish required.
This guide sets out the questions that actually determine the choice.
1. How deep is the repair?
Depth is the first constraint, because every filler has a maximum recommended layer thickness — and exceeding it is the most common cause of cracking, sinking and slow curing.
Shallow surface defects — pinholes, fine cracks, minor unevenness. A fine filler gives a smooth, easily sanded surface with minimal shrinkage. Using a coarse building filler here means more sanding for a worse result.
Deeper holes and damage. A lightweight or building filler with genuine build capacity. Apply in layers, respecting the stated maximum thickness for each, and let each layer dry before the next. Filling a deep hole in one pass is the classic mistake: the surface skins, the core stays soft, and the repair sinks or cracks weeks later.
Large wall and ceiling areas. This is not a repair problem, it is a surface problem. Skim coating with a roller-applied filler and a wide smoothing blade is faster and less physically demanding than working area by area with a knife. It is a different product class and a different method.
2. What is the substrate?
- Plasterboard and interior plaster — most standard interior fillers are suitable. Check whether the product is designed for jointing or for surface filling; they are not the same and jointing compounds are formulated to work with tape.
- Wood — needs a filler with some flexibility, since timber moves with humidity. A rigid filler in a moving joint will crack.
- Metal, and repairs needing real strength — a two-component polyester filler cures chemically rather than by drying, builds thickness in one application and sands hard. It is the right answer for structural or impact-prone repairs, and overkill for a nail hole.
- Exterior surfaces — must be specified for exterior use. Interior filler outside will fail through freeze-thaw cycling, whatever it looks like on day one.
- Highly absorbent or repaired substrates — prime or seal first. Uneven suction pulls water out of the filler at different rates and causes patchy drying.
3. What finish do you need?
The finish requirement is often what settles the choice between two otherwise suitable products.
- Under matt paint on a large flat wall, every ridge and sanding scratch shows. Choose a fine filler that sands to a genuinely smooth surface.
- Under textured or heavily pigmented coatings, minor imperfections are far more forgiving, and a faster-working product is the better commercial choice.
- Where the repair will be visible under sidelight — hallways, stairwells, walls facing windows — treat the finish requirement as one step higher than it looks. Sidelight is unforgiving.
4. What are the site conditions?
Drying and curing times on the tin assume laboratory conditions. Real sites rarely match them.
- Cold or damp conditions extend drying significantly. Water-based fillers rely on evaporation, and in an unheated building in winter that can take far longer than the stated time.
- Two-component fillers cure chemically, so they are less affected by temperature than water-based products — an advantage in cold conditions, but they have a working time that starts the moment you mix.
- Poor ventilation slows everything. Air movement matters as much as heat.
- Sanding too early tears the surface and clogs abrasive. If the filler feels cool to the touch, it is still releasing moisture.
5. How much sanding are you prepared to do?
This is a commercial question, not a technical one, and it is usually underweighted.
Sanding is the slowest, dustiest and most physically demanding part of the job. A filler that applies slightly slower but sands in half the time is often the cheaper choice once labour is counted.
Two things reduce sanding more than any product choice:
- Applying less material. Fill to the surface, not above it. Every millimetre of excess is sanding you have chosen to do.
- Removing ridges while wet. A ridge left at the edge of a repair must be sanded off later. Removing it with the knife takes seconds.
A practical decision path
- Depth? Shallow → fine filler. Deep → building filler in layers. Whole wall → roller-applied skim coat.
- Substrate? Interior board and plaster → standard filler. Timber → flexible. Metal or structural → two-component. Outside → exterior-rated.
- Finish? Matt paint or sidelight → fine, sandable. Textured coating → faster product.
- Conditions? Cold or damp → allow more time, or choose a chemically curing product.
- Then check the technical data sheet for maximum layer thickness, working temperature and overcoating time — and follow it.
The short version
Match the filler to the depth, the substrate and the finish. Respect the maximum layer thickness. Allow real drying time rather than the time on the tin. Fill to the surface rather than above it.
The right filler applied properly needs less sanding, and sanding is where the hours go.
Professional fillers, smoothing blades and sanding systems are available at Kings of Tools.