Coming soon: Trade Chem Exterior Blast is being released shortly. This guide explains how to use it with our House Wash once it's available.
Sodium hypochlorite is the active chemical in bleach, and it's the main chemical behind professional soft washing. Trade Chem Exterior Blast is a commercial grade sodium hypochlorite at 12 to 15%, made for exterior cleaning.
What it does
Sodium hypochlorite is an oxidiser. It breaks down organic growth on contact, so it:
- Kills mould, mildew, lichen, moss and algae
- Lifts the black and green staining they leave behind
- Works at low pressure, so there's no need to blast paint, cladding or roofing
How sodium hypochlorite kills mould
- It forms hypochlorous acid. When Exterior Blast is mixed with water, part of it becomes hypochlorous acid, the main killing agent.
- It gets inside the cell. Hypochlorous acid passes easily through the outer wall of mould, fungi, algae and lichen cells.
- It destroys the cell from the inside. It breaks down proteins, enzymes, the cell membrane and DNA, so the cell can't feed, function or reproduce. The cell dies within minutes.
- It bleaches the staining. It breaks down the black and green pigments, so the staining disappears the same day.
Why growth can come back: sodium hypochlorite is used up as it reacts. On thick growth or porous surfaces it can run out before reaching the deepest roots, and once rinsed off it leaves no protection behind. In shady, damp spots, new growth can return over time.
House Wash or Exterior Blast? Which one do you need?
They do different jobs, and for most houses you'll use them together.
House Wash: the cleaner. House Wash is a concentrated exterior detergent. It contains surfactants and builders that break down and lift:
- General dirt and grime
- Dust, pollen and cobwebs
- Traffic film and grease on cladding
- Light surface staining
It's safe on painted surfaces. On its own, mix up to 1:40 with water, spray the house top to bottom, leave 10 minutes, then hose off.
Exterior Blast: the mould and lichen killer. It kills mould, mildew, lichen, moss and algae on contact, and bleaches out the staining they leave behind.
Used together: a soft wash. Mixing House Wash into Exterior Blast gives you both jobs in one application. House Wash cleans and helps the mix cling to walls, and Exterior Blast kills the growth.
| Your house has... | Use |
|---|---|
| General dirt, dust and cobwebs, no mould | House Wash on its own |
| Black spots, green growth, mould or lichen | House Wash + Exterior Blast |
| Mould on paths, driveways or concrete | Exterior Blast, with a little House Wash to help it cling |
Tip: washing a mouldy house with House Wash alone will make it look cleaner, but the mould isn't killed and can come back faster. Add Exterior Blast to kill it.
For longer protection: follow up with Moss & Mould Remover
Exterior Blast gives you the same-day clean. Moss & Mould Remover keeps working afterwards.
| Exterior Blast | Moss & Mould Remover | |
|---|---|---|
| Speed | Kills and bleaches in minutes | Works slowly over days to weeks |
| Visible result | Same day | Growth dies and weathers off with rain |
| After application | Rinsed off, no ongoing protection | Stays on the surface and keeps working |
| Colour | Can lighten coloured surfaces | Doesn't bleach |
How to follow up:
- Clean with Exterior Blast and House Wash, and rinse thoroughly.
- Let the surface dry completely.
- Apply Moss & Mould Remover at 1:50 for general use, or 1:25 for heavy growth.
- Leave it on, no rinse. Don't apply before rain.
Never mix Exterior Blast and Moss & Mould Remover, or apply one over the other while wet. Together they release toxic chloramine gas.
Moss & Mould Remover is also a good choice on its own for timber decks, coloured concrete and stained surfaces, where bleach could lighten the colour.
Household bleach vs commercial grade
| Household bleach | Exterior Blast | |
|---|---|---|
| Sodium hypochlorite | Typically under 5% | 12 to 15% |
| Made for | Indoor cleaning and laundry | Exterior cleaning and soft washing |
| Additives | Often scented or thickened | No fragrance or thickeners |
How common products compare
| Product | Sodium hypochlorite (as stated on its SDS or label) |
|---|---|
| Janola Bleach | Under 4.9% |
| Domestos Thick Bleach | 3 to 5% |
| 30 Seconds Outdoor Cleaner Concentrate | Under 10%, diluted before use |
| Trade Chem Exterior Blast | 12 to 15% |
Janola and Domestos: made for indoor cleaning, at around a third of Exterior Blast's strength. Domestos is thickened for toilets, so it doesn't spray well.
30 Seconds: made for outdoor use and diluted before use. Exterior Blast lets you mix to an exact strength for each surface.
Cost per job
Shelf price doesn't tell you much. What matters is how much it takes to make the same mix. Approximate cost to make a 10L sprayer of 2% solution, a standard house wash strength:
| Product | Shelf price | Needed for 10L at 2% | Approx. cost per 10L |
|---|---|---|---|
| Trade Chem Exterior Blast 5L | $29.99 | 1.3L | About $7.80 |
| Janola Bleach 2.5L | $7.29 | About 5L (2 bottles) | About $14.60 |
| Domestos Thick Bleach 750mL | $6.49 | About 5L (7 bottles) | About $43 |
One 5L bottle of Exterior Blast makes almost 40L of 2% solution.
Household bleach amounts assume about 4% sodium hypochlorite, as labels state an upper limit rather than an exact figure. Prices are current NZ retail prices at the time of writing and may change.
What it won't do
Sodium hypochlorite doesn't remove oil, grease, rust or general dirt well on its own, and it runs straight off walls. That's why it's mixed with House Wash.
Choosing the right strength
More isn't better. Use the lowest strength that does the job.
| Surface | Strength | Notes |
|---|---|---|
| Painted weatherboards and cladding | 1 to 2% | Soft brush or low pressure, don't let it dry |
| Colorsteel and pre-painted metal roofs and cladding | 2% maximum | 5 minutes maximum, see below |
| Render and plaster finishes | 1% or less | Discolours easily, test first |
| Brick, concrete, paths, driveways and fences | 2 to 4% | Test colour first |
Mix rates: 10L sprayer
| Strength | Exterior Blast | House Wash | Water |
|---|---|---|---|
| 1% | 650mL | 100mL | 9.25L |
| 2% | 1.3L | 100mL | 8.6L |
| 3% | 2L | 150mL | 7.85L |
| 4% (concrete and brick only) | 2.6L | 150mL | 7.25L |
Mix rates: 5L pump-up sprayer
| Strength | Exterior Blast | House Wash | Water |
|---|---|---|---|
| 1% | 325mL | 50mL | 4.6L |
| 2% | 650mL | 50mL | 4.3L |
| 3% | 1L | 75mL | 3.9L |
| 4% (concrete and brick only) | 1.3L | 75mL | 3.6L |
For Colorsteel and metal roofs, use the 2% rate and halve the House Wash.
Using a pump-up sprayer
A 5 to 8L pump-up garden sprayer is the easiest way to apply Exterior Blast on smaller jobs like paths, driveways, fences and single walls.
- Keep one sprayer just for Exterior Blast. Never use a sprayer that has held acids or quat based moss treatments, as leftover residue can react and release toxic gas.
- Use a plastic sprayer. Sodium hypochlorite corrodes metal wands and fittings.
- Rinse after every use. Run clean water through the tank, pump and wand.
- Don't store mixed product in it. It loses strength within a day.
- Release the pressure before opening and after finishing.
Soft wash systems
Most professional soft washers use a soft wash system that mixes the chemical as they spray. A low pressure pump draws from three lines, each with its own metering valve:
- Exterior Blast (sodium hypochlorite)
- House Wash (surfactant)
- Water
You adjust each valve to set the ratio, and the three blend before the nozzle. You can change strength on the spot without mixing a new batch, and the sodium hypochlorite stays fresh in its own drum.
| Strength at the nozzle | Exterior Blast | House Wash | Water |
|---|---|---|---|
| 1% | 8% | Low setting, about 1% | Balance |
| 2% | 16% | Low setting, about 1% | Balance |
| 3% | 24% | Low setting, about 1% | Balance |
| 4% (concrete and brick only) | 32% | Low setting, about 1% | Balance |
Settings are based on Exterior Blast at 12.5%. Valve brands draw slightly differently, so check your result and adjust.
How to apply
- Protect stormwater before you start (see below).
- Wet surrounding plants and gardens with fresh water.
- Apply at low pressure.
- Leave it until the growth lightens, without letting it dry. On Colorsteel, 5 minutes maximum.
- Rinse the surface thoroughly with fresh water, then rinse the plants again.
Colorsteel and pre-painted metal roofs
NZ roofing guidance is specific about metal roofs and cladding:
- 2% sodium hypochlorite maximum. Stronger solutions can damage the paint coating.
- 5 minutes maximum contact time. Don't extend it.
- Apply with a soft broom or low pressure spray.
- Rinse with plenty of fresh water. A low pressure water blaster can be used for the rinse.
- No more than 5 to 7mL of detergent per litre.
- Disconnect any downpipes to a rainwater tank before you start.
Concrete, driveways and paths
Exterior Blast is ideal for black mould, green algae, lichen spots and slippery growth on concrete, pavers, paths and driveways.
- Sweep off loose dirt and leaves.
- Block or cover nearby drains and sumps first. Driveways almost always run to stormwater.
- Wet nearby lawn edges and garden beds.
- Spray on evenly at 2 to 4% and leave until the staining lightens, about 10 minutes, without letting it dry.
- Rinse well with a hose or low pressure water blaster, directing water onto lawn or garden, not the drain.
Exterior Blast won't remove oil or tyre marks. Those need a degreaser.
Surfaces to take care with
- Coloured concrete, exposed aggregate, sealed surfaces and pavers: test first, as it can lighten colour.
- Bare timber: it can bleach and damage the timber.
- Metal fittings like aluminium, brass, gate hardware and garage door bottoms: rinse straight away.
Never mix with
- Acids: releases chlorine gas.
- Quat based cleaners and moss treatments, including Moss & Mould Remover: releases chloramine gas.
Only mix Exterior Blast with water and House Wash.
Keep it out of stormwater
Stormwater drains run straight to streams and the sea with no treatment. Auckland Council doesn't allow any wash water into stormwater, including biodegradable detergents and moss and mould removers, and fines apply.
Before you start:
- Disconnect downpipes and divert runoff onto lawn or garden areas.
- Block nearby drains with drain covers, mats or bungs.
- Use stormwater diversion bags, sand socks or sandbags to hold runoff on site.
- Collect heavy runoff with a wet vac or pump.
- Don't apply when rain is forecast.
Other councils have similar rules, so check with yours.
Storage and shelf life
Sodium hypochlorite loses strength over time, faster in heat and sunlight. Store it cool, shaded and capped, and mix only what you need on the day.
FAQ
Do I need House Wash and Exterior Blast?
If there's mould, lichen or green growth, yes. House Wash cleans, Exterior Blast kills the growth. For dirt only, House Wash on its own is enough.
How do I stop it coming back?
Follow up with Moss & Mould Remover once the surface is rinsed and completely dry.
Will it kill my plants?
It can burn plants if left on them. Wet them before and rinse them after.
Can I use it on my Colorsteel roof?
Yes, at 2% maximum, for no more than 5 minutes, then rinse thoroughly.
Can I use it on my driveway?
Yes, at 2 to 4%. Block nearby drains first, and test coloured or sealed concrete.
Why isn't my mix working?
Either the product is old and has lost strength, or the mix is too weak for the growth. Step up one level, staying within the limit for that surface.
Can I use household bleach instead?
For a small patch, yes. For a whole house you'd need around 3 times as much, at about double the cost.
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House Wash | Moss & Mould Remover | Exterior Blast (coming soon)