Inspired by a helpful YouTube guide. We based this walk-through on the popular UK video "How to Silicone Tricky Gaps" by Charlie DIYte, a well-known British DIY channel with hundreds of thousands of subscribers.
1. Choose the right silicone
Not all silicone sealants are the same. For external window and door frames, you want a low-modulus silicone that flexes with frame movement - standard acetoxy silicone (the type that smells of vinegar as it cures) is fine for most uPVC windows and masonry gaps, but avoid it on natural stone or metal as the acetic acid can stain or corrode.
For bathroom or wet areas near a door, use a sanitary silicone with fungicide. For unpainted timber frames, check the tube says it can be overpainted. Neutral-cure silicones are safer on sensitive materials. Pick a colour that matches your frames - white, grey, clear, and brown are the usual options at a builders merchant.
2. Remove the old sealant completely
This is the step most people underdo. New silicone on top of old silicone does not bond properly - it just peels away within a year. Use a silicone scraper or a Stanley knife blade to cut away the bulk of the old bead, then go back over with a silicone remover spray (Everbuild or similar). Leave it a few minutes and the softened residue scrapes away cleanly.
On plastic frames, work at a shallow angle and let the remover do the softening rather than forcing the blade. Take your time here - it is the foundation of the whole job.
3. Clean and dry the surface
Silicone will not stick to a dusty, damp, or greasy surface. Wipe both sides of the joint with methylated spirits or isopropyl alcohol on a clean cloth and leave it to evaporate. Do this in mild, dry weather if you can - a joint still damp from morning rain will trap moisture and the bond will fail within a season.
If the frame or masonry has any mould, treat it first with a mould remover, leave to dry fully, then clean with meths before applying sealant. Skipping this step just seals the mould in.
4. Mask either side of the joint
Low-tack masking tape either side of the gap makes the difference between a professional result and a messy one. Run a strip of tape parallel to the joint on both the frame and the wall, leaving just the gap exposed. Press the tape edges down firmly so sealant cannot creep underneath.
Take a moment to check the tape lines are straight and parallel. It takes two minutes and saves a lot of cleaning up later. A credit card pulled along the tape edge beds it down well.
5. Load and prepare the cartridge
Cut the nozzle at a slight angle - about 45 degrees - and make the hole a little smaller than the gap you are filling. A nozzle hole that is too wide is harder to control. Pierce the foil seal inside the nozzle with a thin nail or the spike on the gun itself.
Load the cartridge into the gun and squeeze a small test bead onto a piece of card to get the sealant flowing. Wipe the nozzle clean before you start on the actual joint.
6. Apply the bead in one smooth pass
Hold the gun at roughly 45 degrees to the joint, angle the nozzle into the gap, and move at a steady pace while squeezing the trigger. The aim is to fill the gap and lay a slightly proud bead in one continuous pass - stopping and restarting creates lumps at the joins. Work from one end to the other without rushing.
If you need to pause, release the trigger pressure before you stop moving. Keep a damp cloth and a small pot of washing-up liquid diluted in water nearby for the next step.
7. Tool and smooth the bead
Dip a finger in the soapy water and run it firmly along the bead in one smooth stroke, pressing the sealant into the joint and leaving a neat concave surface. Do this while the sealant is still wet - you have a few minutes before it starts to skin. A proper silicone finishing tool gives a consistent result, but a wet finger works well enough.
The key is one pass. Going back over it repeatedly just picks the sealant up and makes a mess. If it goes wrong, you are better off wiping it all off with meths and starting again than trying to rescue it.
8. Remove the masking tape and leave to cure
Pull the masking tape away at an angle before the sealant skins - ideally within two or three minutes of tooling. Pull towards the joint rather than away from it, which keeps the edge clean. Leave it too long and the sealant will skin to the tape and tear when you remove it.
Most silicones are touch-dry within an hour and fully cured in 24 hours. Keep the joint dry during curing. Acetoxy types give off vinegar fumes while they cure, so ventilate the room. Check the tube for the manufacturer's specific cure time before getting the joint wet.
When to call a handyman
If the gap is wider than about 10mm, standard silicone is not the right fix on its own - gaps that size need backing rod or expanding foam first, then sealant over the top. If the frame is rotting or moving with the seasons, sealing over the problem just hides it for another year. And if the gap has opened up because a lintel has shifted or the wall has settled, that is worth getting looked at before any cosmetic sealant goes on.
Applying sealant in an awkward spot - a high window, an external soffit, anywhere that needs a ladder - is also worth calling someone in for. It is one of those jobs where working quickly and confidently makes all the difference, and that is hard when you are balancing above head height.
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Richard covers sealant work, draught-proofing, and general repairs around Sandwich and East Kent. No call-out fee on standard jobs - just get in touch and describe what needs doing.
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