What Not to Pressure Wash

1. The Never List

These are the surfaces where no pressure setting is the right setting. Every one produces damage that costs more to repair than the cleaning was worth.

2. Wood, Paint and Soft Materials

These can be pressure washed, carefully, and they are where most avoidable damage actually happens because the surface looks robust and is not.

• Peeling or failing paint strips in sheets, which turns a cleaning job into a scraping and repainting job.
• Wood decking furs and raises the grain above 1,800 PSI or wherever the wand pauses, and only sanding fixes it.
• Cedar and other soft timbers mark far more easily than pressure-treated pine at the same setting.
• Composite decking scars and voids its warranty on many products, so check the manufacturer figure first.
• Painted wood siding lifts at edges and seams, especially where the paint is already tired.
• Aluminium siding dents and chalks, and once the finish is chalked it cannot be restored by washing.
• Outdoor fabric, cushions and shade sails delaminate and tear at surprisingly low pressure.

The composite decking point catches people out most often. It looks like a hard, maintenance-free surface, and many manufacturers specify a low maximum pressure with a wide tip, above which the warranty is void. Look up the figure for your product rather than assuming, because they vary widely.

On any wood, the technique matters as much as the number. Keep the wand moving in steady overlapping passes, work with the grain, and let a deck cleaner do the chemical work first. Our guide to deck cleaners covers the products that make low pressure sufficient.

3. Anywhere Water Can Get Into the Building

This is the category that produces the most expensive repairs, because none of it shows on the day. Water driven into a wall has nowhere to dry, and the consequences appear months later as damp, mould or rot.

• Never spray upward under lap siding. The overlap sheds rain from above and offers no protection against a jet from below.
• Never drive water into soffit vents, which open directly into the roof space.
• Do not spray at window and door trim junctions, where flashing and sealant are the only barrier.
• Avoid utility penetrations: dryer vents, cable entries, hose bib escutcheons, meter boxes.
• Do not wash over loose or cracked siding boards, which are simply holes in the envelope.
• Keep away from gutter joints and seams, which separate under a directed stream.
• Do not spray into weep holes on brick, which exist to let water out rather than in.

4. Masonry That Is Older Than It Looks

Brick, stone and pavers all tolerate real pressure. What sits between them frequently does not, and the joints are what hold the structure together.

The warning sign column is the useful one. In every case the material tells you before the damage is serious, and stopping at that point costs nothing. Sand in the runoff on a paver patio means you are washing out the joints, and continuing means re-sanding the whole area afterwards.

If you do wash out paver joints, that is a repair rather than a disaster, and it is worth doing properly. Our guides to using polymeric sand and why polymeric sand washes out cover the reinstatement.

5. Vehicles, Machinery and Equipment

Cars can be pressure washed safely, and there are specific parts of them that cannot. The same applies to garden machinery and outdoor equipment.

• Engine bays are weatherproof against rain, not a directed jet. Water in connectors, sensors and modules causes intermittent faults that are miserable to diagnose.
• Radiator and condenser fins flatten under pressure, and a flattened fin is lost cooling capacity.
• Existing stone chips and peeling lacquer will lift further, because the stream gets under the edge.
• Badges, trim strips and wraps are held on with adhesive that a jet will find the edge of.
• Wheel bearings, CV boots and suspension bushes should not be blasted at close range.
• Lawnmower and generator engines have air filters, ignition components and bearings that dislike direct water.
• Bicycle and motorcycle bearings and seals are easily driven past, which shortens their life considerably.

For bicycles in particular the standard advice among mechanics is a hose rather than a pressure washer, because the water goes straight past hub, bottom bracket and headset seals and washes the grease out. The bike looks immaculate and the bearings are dry, and the second thing is the expensive one.

Where a car is concerned, the safe approach is well established: under 1,500 PSI with a 40 degree tip, two to three feet back, foam and dwell time rather than force. Our guide to washing a car properly covers the whole method.

6. People, Pets and Plants

Worth stating plainly, because the risk here is routinely underestimated on electric machines specifically.

Electric machines are gentler than gas, not harmless. A stream at even 1,500 PSI can cut skin, and a pressure washer laceration injects water and debris under the surface, which is why these injuries are treated far more seriously in an emergency department than they look. They can require surgical cleaning.

• Never point a wand at anyone, and never hold a hand in front of the nozzle to check flow.
• Wear eye protection and closed-toe shoes; debris comes back off the surface at speed.
• Keep the trigger locked when repositioning, and keep children and pets well clear of the work area.
• Never wash a pet with a pressure washer, at any setting, including a power cleaner.
• Pre-wet plants and shrubs before using any detergent, and rinse them thoroughly afterwards.
• Never operate from a ladder; the recoil is exactly the force that causes falls.
• Watch your footing, because the surface you are cleaning becomes slippery as you clean it.

7. Soft Washing: What to Do Instead

Most items on this list still need cleaning. The alternative is not leaving them dirty; it is soft washing, which is what professionals use on anything fragile.

Soft washing means applying cleaning chemistry at roughly garden hose pressure and letting it do all the work, then rinsing gently. It is slower and it removes organic growth more thoroughly than pressure does, because algae and mildew die chemically rather than being knocked off.

The AC entry is worth a note, because it is a job people do want to do. Condenser fins are thin aluminium and flatten under any real pressure, and flattened fins reduce airflow and cooling efficiency permanently. Use a garden hose, spray from the inside of the coil outward so debris exits the way it came in, and turn the unit off at the disconnect first.

The general principle holds across this whole list: chemistry plus dwell time removes organic soiling better than force, and it does it without finding the weaknesses in what you are cleaning. Our guide to using detergent properly covers how to apply it through the machine you already own.