What is the difference between silicone and acrylic sealant?

Getting this wrong is one of the most common decorating mistakes, and it is only discovered after the sealant has been applied and painted or has failed in a shower.

Step by step

1. Ask the deciding question first.Does this joint need to be painted, or does it need to be waterproof? Painting means acrylic. Water means silicone. That single question resolves nearly every case.
2. Use silicone anywhere wet.Around baths, showers, sinks, worktops and external joints. It is genuinely waterproof, stays flexible for years, and resists mould better — especially the sanitary grades with fungicide.
3. Accept that silicone cannot be painted.Paint beads up and peels off it. There is no primer that reliably fixes this. If a joint must be painted, silicone is the wrong product regardless of everything else.
4. Use acrylic — decorator's caulk — for painted joints.Skirting to wall, architrave gaps, coving joints, small cracks. It takes paint perfectly and is designed to disappear under it.
5. Know acrylic's limits.It is far less flexible and it shrinks slightly as it dries. In a joint that moves much, or anywhere that gets wet, it cracks and lets water through.
6. Choose the right silicone grade.Sanitary silicone with a fungicide for bathrooms and kitchens. General-purpose for other internal joints. Frame or exterior grade for outside. They are not interchangeable.
7. Consider hybrid sealants for the awkward cases.MS polymer products are flexible, waterproof and paintable. More expensive, and the right answer for a joint that genuinely needs all three.
8. Prepare the joint properly.Remove every trace of old sealant, then clean and dry thoroughly. New silicone will not bond to old silicone, and that is the single most common reason a reseal fails.
9. Apply in one continuous pass.Cut the nozzle at 45 degrees to roughly the bead width you want, and move steadily. Starting and stopping creates weak points and an uneven line.
10. Tool it once.A wet finger or a shaping tool, one pass, immediately. Repeatedly reworking silicone drags it out of the joint and leaves a poor seal.
11. Fill deep gaps in stages with acrylic.Acrylic shrinks, so a deep gap filled in one go sinks and cracks. Two applications, letting the first dry, gives a flat result.
12. Fill a bath before sealing it.Fill it with water, seal, and let it cure before draining. The bath drops under load, and sealing it empty means the joint is stretched every time it is used.

Tips

• Paintable or waterproof — pick one. That question answers this in five seconds.
• New silicone does not stick to old silicone. Every trace has to come out.
• Fill the bath before sealing round it. It settles under the weight and pulls the joint apart otherwise.

Common mistakes

• Using acrylic caulk around a bath or shower — It is not waterproof and has little flex. It cracks and lets water into the wall, which is discovered much later as damp.
• Trying to paint over silicone — Paint beads and peels off it, and no primer reliably solves it. If the joint needs painting, use acrylic or a hybrid.

If it doesn't work