Domakihub How to Fill Gaps Before Painting: A Room-Ready Guide
Choose the right filler, prep the surface correctly, and get a finish that holds.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
Whether you are refreshing a holiday villa between seasons, renovating a city apartment, or maintaining a hotel room, gaps and cracks in walls are the most common thing that gets painted over too quickly — and the first thing guests or inspectors notice when it fails. The video above walks through the hands-on technique; the steps below give you the full decision tree so you pick the right product for the right gap and avoid callbacks. Greece's climate — heat, humidity swings, and seasonal temperature cycling — makes surface prep more critical than in cooler, more stable northern European markets.
Why Gaps Appear — and Why It Matters Which Kind
Gaps fall into three broad categories: hairline shrinkage cracks in plaster or render (the most common), movement gaps at junctions between different materials (wall-to-skirting, wall-to-door frame, window reveal to masonry), and structural cracks that show differential settlement or ongoing movement. The first two are DIY territory with the right product. The third — a crack wider than about 3 mm that runs diagonally, follows the mortar joints in a block wall, or keeps reappearing — needs assessment by a civil or structural engineer before any decorative work is done. Painting over a structural crack does not fix it; it just hides a problem that will cost more to address later.
Damp-related gaps deserve a separate mention. If a crack is stained, the plaster sounds hollow, or efflorescence (white salting) is present on the surface, there is a moisture source behind the wall. Filling and painting over damp plaster will cause the new filler and paint to fail within months. A licensed waterproofing contractor must address the root cause first. This is especially relevant in basement-level units, older stone buildings in island locations, and ground-floor spaces in areas with high water tables.
Choosing the Right Filler
Interior hairline cracks and small holes (up to about 5 mm wide) are best handled with a fine-surface interior filler — the ready-mixed acrylic type sold in tubs dries faster and sands easily, while the powder type (mixed with water) is cheaper for larger volumes and can be built up in layers for deeper voids. Neither is flexible, so they are wrong for movement joints.
Movement gaps — anything at the junction of two different materials, or around window and door frames — need a flexible, paintable acrylic sealant (sometimes called decorator's caulk). This stays slightly elastic after curing so it can accommodate the seasonal movement that re-opens rigid filler within a year. Silicone sealant is not suitable here: most silicone formulations cannot be painted over and pick up dirt badly. Confirm the label says 'overpaintable' before buying.
Step-by-Step: Filling Gaps Before Painting
- 1
Clean and open the crack
Rake out any loose material from the crack or gap using a scraper, old screwdriver, or caulking tool. Remove dust, flaking paint, and any old filler that has debonded — tapping with a screwdriver handle reveals hollow spots. Wipe the area with a barely damp cloth and let it dry completely. Skipping this step is the single most common cause of filler failure: new product stuck to dust or old loose material has nothing solid to grip.
- 2
Dampen deep voids and apply primer if needed
For voids deeper than about 10 mm, mist the interior of the hole lightly with water before applying powder filler — dry plaster absorbs moisture from the mix too fast, causing shrinkage and cracking as it cures. For very porous or chalky surfaces, apply a coat of diluted PVA bonding agent (typically 1 part PVA to 5 parts water) and let it become tacky before filling. This step is often omitted on fresh plaster repairs and leads to the filler popping out months later.
- 3
Apply filler in thin layers
Press filler firmly into the crack with a flexible filling knife, working across the gap to force product in rather than just bridging over it. For gaps deeper than about 5 mm, build up in two or more passes — each layer no more than 5 mm — allowing each coat to dry before adding the next. Overfilling slightly is correct; you want a small proud ridge that can be sanded flush. Trying to fill deep voids in one pass causes the centre to shrink and crack as it dries.
- 4
Sand smooth when fully dry
When the filler is completely dry (colour changes from dark to uniform pale, no cool feeling to the touch), sand with 120-grit abrasive paper wrapped around a sanding block — not bare fingers, which follow the contour and miss the high spots. Feather the edges into the surrounding wall surface. Wipe away dust with a dry cloth. For movement joints filled with acrylic sealant, do not sand; instead, tool the wet sealant flush with a moistened finger and leave it to cure fully (typically 24 hours) before painting.
- 5
Spot-prime before the finish coat
Apply a coat of primer or diluted first-coat paint over every filled area before the full paint system goes on. Filled patches are more absorbent than surrounding plaster and will show as dull 'flat' spots through the top coat — a phenomenon known as picture-framing — if not primed first. A small brush is fine for spot-priming. Once dry, the surface is ready for the full paint sequence. In high-humidity Greek environments (bathrooms, kitchens, coastal properties), use a moisture-resistant primer and confirm your topcoat is rated for the exposure level.
Rather have a pro handle it?
Get a vetted renovation pro in your city — one number, booked in EN/EL/DE.
FAQ
- Can I paint over filler the same day?
- With thin applications of ready-mixed acrylic filler in warm, dry conditions you may be able to prime and paint within 4-6 hours, but the safest practice is to wait until the filler is fully dry — at least overnight for anything deeper than a hairline. In humid Greek summers, drying can be faster; in winter or in poorly ventilated rooms it slows significantly. Painting over filler that is still damp traps moisture, causes cracking, and can lead to mould growth behind the paint film.
- What is the difference between filler and sealant, and when should I use each?
- Filler (also called patching compound or spatula filler) is rigid when dry, sands easily, and is designed for static voids in flat surfaces — holes, dents, and cracks in plaster or drywall. Sealant (decorator's caulk) stays flexible after curing and is designed for movement joints — the gap between a wall and a door frame, a skirting board, or a window reveal. Using rigid filler in a movement joint means the crack re-opens within one heating or cooling cycle. Using flexible sealant on a large void in plaster gives a surface that cannot be sanded flat. Match the product to the type of gap.
- A crack in my wall keeps coming back after I fill it. What should I do?
- A recurring crack almost always means ongoing movement, which rigid filler cannot accommodate. First, identify the cause: minor seasonal movement at a material junction is normal and sealant will handle it; a crack that widens, shifts, or appears elsewhere in the same wall pattern needs professional assessment. In Greece, a licensed civil engineer (Πολιτικός Μηχανικός) can inspect the structure and issue a written opinion. If the building has had underpinning work, a change of use, or additions to upper floors, do not attempt to dismiss recurring cracking as cosmetic — it may require a building permit to remediate.