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Sealants and Joint Fillers

A sealant closes joints and accommodates material movement without breaking. The choice depends on three factors: whether the joint moves a lot or a little, if water is present, and if you plan to paint over it afterward. That last point immediately rules out silicone.

Which sealant to choose
Silicone cannot be painted. This is the most common mistake on-site: a joint is sealed with silicone, and then the paint doesn't adhere, it retracts and forms beads. If the joint will be painted, use acrylic or MS polymer.
Type Paintable Movement Typical Use
Acrylic Yes Low Cracks, skirting boards, frames, dry interiors
Neutral Silicone No High Bathroom, kitchen, glass, sanitaryware
Polyurethane Yes High Structural joints, facade, exterior
MS Polymer Yes High Seals and bonds, solvent-free, almost all substrates
Correctly sizing the joint

An incorrectly sized joint will break even if the product is correct. The sealant needs to be able to stretch, and for that, it needs depth and proportion.

  • Indicative proportion: depth should be half of the width, for movement joints.
  • Use a backer rod (polyethylene) to control the depth.
  • The sealant should only adhere to two surfaces, not the back. If it adheres to three, it cannot stretch and will tear.
  • Check the product's admissible movement (usually ranges from 12.5% to 25%).
  • For glass, mirrors, marble, or natural stone, use neutral silicone: acetic silicone attacks these materials and darkens the edges.
Clean application
  • 1 → Clean and degrease the joint. Completely remove old sealant; do not cover it.
  • 2 → Cut the nozzle at a bevel to the width of the joint, no more, no less.
  • 3 → Apply by pushing the bead forward, in a continuous, non-stop pass.
  • 4 → Smooth before it skins over. With masking tape on both sides, it will be perfect: remove the tape while wet.
  • 5 → Allow proper curing. Silicone cures in depth about 2 mm per day: do not wet prematurely.
Related: silicones, polyurethane foam, adhesives, repair fillers, and application guns. Questions? WhatsApp +34 621 20 71 54.

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