Yes, you can use a heat gun to soften old glazing putty — and it's one of the most effective methods for removing hardened putty from timber window frames. However, there are real safety risks involved, particularly around lead paint, glass cracking, and fire, so it pays to understand the correct approach before you start.
Why Old Glazing Putty Gets So Hard
Traditional linseed oil putty cures over time by oxidising, and in older NZ homes — especially those built before the 1980s — it can become rock-hard and brittle over decades. Trying to chip it out cold with a chisel often risks cracking the glass or damaging the rebate in the timber frame. Gentle heat softens the oils still present in the putty, making it pliable enough to remove cleanly.
The Lead Paint Risk — Read This First
Before you reach for the heat gun, check whether your window frames have lead-based paint. Lead paint was commonly used in NZ homes built before 1980. Heating lead paint — even with a heat gun — can release toxic lead fumes and fine particles.
- Use a lead paint test kit (available from Bunnings or Mitre 10) before starting.
- If lead paint is present, wear a properly rated respirator (P2 minimum), disposable coveralls, and gloves.
- Keep children and pregnant women well away from the work area.
- Dispose of scrapings carefully — don't sweep or vacuum without a HEPA filter.
WorkSafe NZ has detailed guidance on working safely with lead paint in older homes. If there's significant lead paint present, consider engaging a professional rather than DIYing.
Equipment You'll Need
- Variable-temperature heat gun (not a hair dryer — it won't get hot enough)
- Stiff putty knife or hacking knife
- Safety glasses and heat-resistant gloves
- P2 respirator (essential if any paint is present)
- Drop cloth to catch scrapings
- Bucket of water or fire extinguisher nearby
Step-by-Step: How to Soften Glazing Putty with a Heat Gun
1. Set the Right Temperature
Keep your heat gun between 200°C and 300°C. Higher settings risk thermally shocking the glass, which can cause it to crack or shatter — especially in cold NZ winters when glass is already cool. Never direct the heat gun at the glass itself; focus the airflow on the putty and timber rebate only.
2. Keep the Gun Moving
Hold the nozzle 50–75 mm from the putty and keep it moving in slow, steady passes. Dwelling in one spot too long can scorch the timber, crack the glass, or ignite old paint layers.
3. Test for Softness
After 20–30 seconds of heating a section, press your putty knife into the putty. When it yields easily, it's ready to remove. Work in short 150–200 mm sections rather than trying to heat a whole pane at once.
4. Remove the Putty
Use the hacking knife or putty knife at a low angle to lever and slice the softened putty away from the rebate. Remove any glazing sprigs (the small metal pins holding the glass) with pliers before re-puttying.
5. Clean Up the Rebate
Once the putty is removed, sand or scrape the rebate back to bare timber and apply a coat of linseed oil or timber primer before fitting new putty. This helps the new putty bond and last longer.
Fire Safety
Heat guns are a common cause of house fires during renovation work. Keep these points in mind:
- Never leave a heat gun unattended or resting face-down.
- Have water or a dry powder extinguisher within reach.
- Check the wall cavity and surrounding timber for smouldering after you finish — fires can start in hidden spots and take hours to develop.
- WorkSafe NZ recommends checking for heat damage for at least an hour after finishing hot work.
Alternatives to a Heat Gun
If you're uncomfortable using a heat gun — particularly around lead paint — consider these options:
| Method | Pros | Cons |
|---|---|---|
| Heat gun | Fast, effective | Lead fume risk, fire risk |
| Chemical putty softener | No heat risk | Slower, may need multiple applications |
| Cold chiselling | No fumes | Risks cracking glass, slow |
| Professional glazier | Safe, efficient | Costs more |
Chemical putty removers are available from Mitre 10 and PlaceMakers — always follow the manufacturer's instructions and ventilate well.
NZ Building Code Considerations
Simply replacing glazing putty in an existing window is generally exempt from building consent under Schedule 1 of the Building Act, as it's routine maintenance. However, if you're replacing the glass itself — particularly upgrading to safety glass or double glazing — check with your local council, as consent requirements can vary. MBIE Building Performance has useful guidance on what work requires consent.
If your windows are in a rental property, note that Tenancy Services Healthy Homes Standards may apply to draught stopping and insulation, which can affect how you approach window maintenance.
When to Call a Professional
Call a qualified glazier if:
- The glass is cracked, broken, or larger than you can safely handle alone.
- You suspect significant lead paint and aren't equipped to manage it safely.
- The window frames are rotten or structurally compromised.
- You're dealing with safety glass, heritage joinery, or double-glazed units — these need specialist handling.
- You're unsure about any step of the process.
A professional glazier has the tools and experience to remove old putty, reglaze, and re-putty efficiently and safely. You can find qualified local glaziers on tradedirectory.co.nz.
Window & Glass Association
