What should I use to smooth a sealant bead?

An untooled bead often is not actually stuck. Sealant leaves the nozzle as a rounded ridge that touches down along two lines, and the pressure of tooling is what forces it into positive contact with both faces and eliminates the small voids at each edge. That is why a bead that looks perfectly complete can peel away in one strip a few months later.

What you'll need

• a set of profiled sealant tools, or a rounded plastic spatula
• clean water in a small container, or a proprietary tooling solution
• a cloth or kitchen roll
• masking tape
• nitrile gloves

Step by step

1. Have everything ready before you gun the bead.Tool, water, cloth and a bin bag for the waste, all within reach. The usable window for tooling is a couple of minutes and it is shorter in warm dry conditions. Fetching a spatula mid-run ruins the bead.
2. Tool in one continuous pass along the run.One direction, one pass, firm and steady. Repeated passes drag material out of the joint and thin the middle of the bead. Long runs can be tooled in sections, but each section is one pass.
3. Press firmly enough to feel the joint faces.The point is to push the sealant sideways into both surfaces. Light tooling only shapes the surface and leaves the voids at the edges intact. It should feel like pressing, not like stroking.
4. Aim for a shallow concave profile.Slightly hollow, not flat and not proud. A concave surface puts the sealant in even tension when the joint opens rather than concentrating the stretch at the thin edges, and there is no ridge for dirt to lodge against.
5. Choose the tooling medium deliberately.Clean water works for most sealants and leaves nothing behind. A proprietary tooling solution gives the slickest finish and is formulated not to interfere with adhesion. Dilute washing-up liquid gives a lovely finish and leaves a surfactant film, which is fine on the exposed surface and unhelpful if it runs into the joint edges.
6. Never tool dry.A dry finger or a dry tool drags the sealant, leaves ridges and picks material up rather than smoothing it. This accounts for most first attempts looking obviously homemade.
7. Use a shaped tool rather than a fingertip where the finish matters.A profiled tool sets a consistent bead width and radius along the whole run, which a fingertip cannot. A set of them costs very little and is the single cheapest improvement to the result.
8. Pull masking tape immediately after tooling.While the sealant is still wet, at a shallow angle, away from the bead. Tape left until the sealant skins tears the edge you have just formed.
9. Stop if it has started to skin.Once a skin has formed, tooling tears it and drags the skin into the wet material underneath, which looks worse than leaving it alone. If a section has skinned, leave it and accept the finish, or cut it out and redo it.

Tips

• A wet finger is a perfectly good tool for a small job. Wet it in clean water, use one firm pass, and wipe it on a cloth between passes rather than back onto the bead.
• The excess that gathers on the tool needs somewhere to go. Wipe it onto kitchen roll after every pass, or it gets dragged back onto the joint.
• Cold sealant is stiffer and harder to tool. A cartridge kept at room temperature makes a visible difference to the finish.
• For an internal corner, work from the corner outwards in both directions rather than trying to tool through it, which always leaves a smear.
• On a very long run, cut the nozzle slightly smaller and do it in two lengths with a deliberate overlap rather than trying to gun and tool six metres in one go.

Common mistakes

• Going back over the bead a second and third time — Each pass removes material and thins the centre. The bead ends up with too little section to stretch and a torn surface.
• Tooling with a dry finger — It drags, sticks and leaves ridges, and it lifts sealant out of the joint rather than pressing it in.
• Leaving the masking tape until it has cured — The skin tears at the tape line and takes the clean edge with it. Tape comes off while the bead is wet.
• Flooding the joint with soapy water — Surfactant running into the joint edges before the sealant has bonded there is a release agent in exactly the wrong place. Wet the tool, not the joint.

Questions people ask