How to replace an old window pain with a hacking knife?

29th February 2020

How to replace an old window pain with a hacking knife

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		Hacking knives were originally intended to remove the hard ageing putty from around old, wood-framed windows. To replace an old or damaged window pain you will need several other tools along with your hacking knife.

		Other tools you may require
		Other tools that might be required are: hammer, duct tape, flat head screwdriver, wire brush, sand paper, putty knife, paint brush, undercoat, glazing pins and long-nose pliers.

		Step 1 – Tape glass Place duct tape over any cracks of broken glass. This is best done on both sides to help prevent the glass separating as you cut the putty away and remove the glass.

		Step 2 – Remove old putty Using the hacking knife and hammer, start at a 90 degree angle to the edge of the window frame and hack into the putty, using firm strikes of the hammer on the back of the blade.



		Once you have chipped out a section of putty, keep the blade of the hacking knife flat and parallel with the edge of the window frame. Use firm strikes of the hammer on the back of the blade to hack out the remaining putty from around the window.

		Be careful not to cut into the wooden frame of the window with the hacking knife and watch for glazing pins as you hack out the putty. Although the hacking knife is strong enough to cut through these, doing so may damage the glass or window frame.

		Glazing pins are just the same as panel pins, while glazing points are flat pieces of metal with a triangular point and two little bent up flaps that allow you to press them into a wooden window frame.

		Step 3 – Clean and remove glass If glazing pins or points have been used to hold the old pane of glass in the frame, use the flat-head screwdriver and long-nose pliers to lever and pull these out. Once all these are removed, you can remove the old pane of glass from the window.

		Step 4 – Preparation Prepare the frame for the new pane of glass by brushing out all the remaining dust and dirt with a soft brush. Then paint the frame with an oil-based undercoat. This will help to seal the wooden frame and prevent it from drawing out and absorbing the oils from the putty, which can cause the putty to crack.

		Step 5 – Replace glass pane Take a small amount of glazing putty and press it all the way around the edge of the window frame where the glass will sit (this is known as the rabbit). This will create a good seal around the glass that will help prevent it rattling and condensation occurring.



		Aim for a bed of putty of 1-3mm between the glass and the frame. Place the new glass pane into the window frame and press it down against the putty, making sure the pane sits evenly all the way around. If necessary, place spacers under the glass to centre it. Matches and the legs of tile spacers are ideal for this.

		Then secure the pane in place by pressing or lightly hammering a couple of glazing pins or points into the frame on each side. You may find this easier if you hold them with a pair of long nose pliers.



		Step 6 – Reseal with putty Take some more glazing putty and firmly press it in around the edges of the window frame using a stiff putty knife. Once the putty is in place it can be made to look neat.

		To do this, start at one corner of the window frame with your putty knife at a 45 degree angle and slowly draw it along the edge of the window frame, smoothing out the putty until you reach the next corner. When you reach the corner draw the putty knife away from the glass, towards the corner of the window frame to give a clean sharp corner to the putty.

		Any excess putty that squeezes out can be scraped off afterwords. Fill any gaps that may appear with more putty and finish off by making a final pass over with a whetted putty knife blade to give a smooth finish.

		Repeat the process of scraping off excess putty that may have squeezed out on the inside of the window frame. This should be done at a 90 degree angle to the glass and not 45 degree like on the outside, so as not to leave any putty visible as you look out the window.

		Allow the putty to dry before painting. How long this takes can vary depending on the type of putty you have used and the temperature, but it’s not uncommon to have to wait several weeks before painting.