How do you use a silicone gun?

Loading it the first time

A new gun looks jammed until you know where the release is. It's the lever or catch at the back, where the rod comes out. Press it with your thumb and hold it down, and the rod slides all the way back. Let go and it locks again.

With the rod back, put the cartridge in nozzle end first, through the front of the frame. Push the plunger forward until it meets the back of the tube. Then pump the trigger a couple of times until you feel it push back. That resistance means the plunger is against the cartridge and the next squeeze moves sealant.

Some guns have no thumb lever and use a ratchet at the end of the rod instead, which you turn. If yours won't pull back, check which kind you've got before you force it.

Opening the cartridge

A new cartridge is sealed at the tip of its threaded spout, the bit the nozzle screws onto. Cut the very end off with a sharp knife, and leave the thread itself alone so the nozzle still has something to grip.

Some cartridges have a second seal just inside. If nothing comes out once it's loaded and the nozzle is on, check for one. Push a nail or a pin down through the spout to break it.

Then screw the nozzle on by hand until it stops.

Cutting the nozzle is the one step you cannot undo

The hole you cut sets the size of the bead. Cut close to the tip and you get a thin line. Cut further back and it's thicker. The instructions agree on the rest: cut at about 45 degrees, with the hole about the width of the gap you're filling, or a touch bigger.

Here's the catch. You can always cut more off. You can never put it back. So start small, run a few centimetres onto a bit of cardboard, and only go wider if the bead is too thin for the joint. Squeezing harder through a tiny hole isn't the fix and a hole cut too wide gives you a fat, messy bead.

How big the bead should be for a bath, a sink or a worktop, and how to get it smooth, is in the guide to applying silicone sealant. Getting the hole right first is what makes that part easy.

Running the bead: push it, at a steady pace

Hold the gun at about 45 degrees to the joint, squeeze gently and move at one steady speed. Even pressure and even speed give you an even, unbroken bead.

Which direction to move is where the instructions disagree. Some say draw the gun towards you. Others say push the sealant ahead of the tip. Push it. The nozzle then presses the sealant into the gap in front of it. One building products maker puts the problem with pulling plainly: pull too quickly and "hardly any sealant is able to penetrate the joint". If you can only see the line by pulling, pull, but go slower.

Do one joint in one go. Silicone skins over, and once it has you can't smooth it. Dow's instructions for its tub and tile silicone give you 5 to 10 minutes to smooth it. Run the whole length, then smooth it straight away. When you can use the shower is a separate clock, and how long silicone takes to dry has it.

Why it keeps coming after you let go

This is the bit that catches everyone out. When you let go of the trigger, the rod doesn't move back. It stays pressed against the plunger, so the cartridge is still squeezed and the sealant keeps oozing out until something takes that pressure off.

That's how most guns are built. A 1996 patent for a dripless gun describes ordinary ones in a single line: they "maintain full pressure when the trigger is released", and you have to press the release lever to end the bead. In the gap before you do, it says, extra sealant comes out and "a messy and uneven bead often results".

So make the release a habit. Press it whenever you stop, to move your hand, to wipe the nozzle or at the end of the joint. That takes the pressure off and the ooze stops. On a ratchet gun, turn the rod instead. A dripless gun does this for you by letting the rod slip back a fraction when you let go, which is all "dripless" means.

When you've finished, the cartridge needs closing or the sealant in the spout sets. Screw-on caps that plug the spout in place of the nozzle are the tidy way to do it, and how to stop silicone drying in the tube covers how long an opened one lasts.