What is the difference between sealant, filler and adhesive?
Three products that look identical in the tube and do entirely different jobs — and what happens when you pick the wrong one.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 2 min
- Safety
- caution
Short answer
Filler hardens and can be sanded and painted, and it cracks if the gap moves. Sealant stays flexible and cannot be sanded, so it suits joints that move. Adhesive sticks things together. Using filler where you need sealant is the classic mistake.
The difference that matters is movement. Anything that expands, contracts or gets wet needs a flexible product; anything static that will be painted needs a rigid one. Choosing on that basis gets it right nearly every time.
Safety
Step by step
- Ask whether the gap moves.Around a bath, a window frame, or where two different materials meet — yes, it moves. Inside a wall, a hole in plaster — no. This single question decides most of it.
- Use filler for static gaps you will paint.Cracks in plaster, holes from screws, dents in wood. It hardens, sands flush and takes paint. It will crack if the gap moves.
- Use flexible decorator's caulk for painted joints that move slightly.Where skirting meets wall, around door frames, ceiling-to-wall corners. It flexes with seasonal movement and it is paintable, which is what distinguishes it from silicone.
- Use silicone sealant for wet areas.Baths, showers, sinks, worktop edges. Waterproof and flexible. It cannot be painted, so match the colour instead — this catches people out constantly.
- Use adhesive to hold things together.Grab adhesive for fixing things to walls without screws, wood glue for joints, PVA for porous materials, epoxy for a strong structural bond.
- Check whether it is paintable before buying.The most consequential detail. Silicone cannot be painted and paint applied over it beads and peels off, so a mistake here means cutting it all out.
- Match the product to the material.Some sealants do not adhere to plastics, some adhesives melt polystyrene, and some products are unsuitable for natural stone. The tube says.
- Prepare the surface.Clean, dry, and free of old product and dust. Nearly every failure of any of these three is a surface preparation failure rather than a product one.
Tips
- Mould-resistant silicone in bathrooms is worth the small extra. Ordinary silicone in a shower goes black within a year or two.
- A wet finger or a proper smoothing tool gives a neat sealant bead. A dry finger drags it and leaves a mess.
- Buy the smallest tube that will do the job. Opened tubes set solid within weeks whatever the cap claims.
Questions people ask
Can I paint over silicone sealant?
No. Paint beads and peels off silicone. If a joint needs painting, use flexible decorator's caulk instead — it flexes and takes paint.
What is the difference between filler and caulk?
Filler hardens and can be sanded, which suits static gaps. Caulk stays flexible, which suits joints that move slightly, like skirting against a wall.