Water Blasting
Pressure washers are used by most painters for exterior pre-paint preparation. Pressure washers can clean off the dirt and grime, kill mold and mildew, remove paint chalking and loose peeling paint. The term used for pre-paint preparation with a pressure washer is “water blasting”
Be careful of water damage and electricity
Don’t spray water into the house through poorly sealed windows and doors. Don’t spray water through attic vents, soffit vents or other open vents. Be careful of all electrical outlets and light fixtures. You are spraying water, which is a conductor of electricity, avoid any electrical elements on the house. If possible, turn off the electricity to the outlets and fixtures on the exterior via the circuit box. Protect outlets and fixtures with plastic and duct tape to prevent water intrusion. Above all else, avoid spraying near any outlet or electrical fixture, hand clean these areas with dry cleaning methods. With aluminum and vinyl siding you need to be careful not to inject water behind the siding through the seams and vent cuttings.
Scaring the substrate
Another caution to observe when water blasting is the problem of damaging the substrate that you are cleaning. Water under pressure has enough force to cut into wood and rip it up. Hard surfaces can take a harder blast, softer wood surfaces must be cleaned with more moderate pressure. Cement, metal (aluminum, steel, , etc.), vinyl, and (most) brick, can take a fairly hard blast. Wood, and especially bare wood, requires a lot of vigilance and care to avoid scaring. Drop your pressure, and widen your tip with soft surfaces.
Cleaning off chalk and dusty dirt
Old paint will fade and form a chalk facing on exterior surfaces. Paint does not adhere well to chalk (latex paint especially), and it must be removed. (with pre-1978 houses that may contain lead paint, you should not water blast the chalk off. Bind the chalk down with Emulsa-bond instead).
Water pressure alone will usually remove old paint chalk and environmental dust / dirt. Use a 15 degree tip on hard surfaces like aluminum siding or cement block at 2000 -3000 psi. You can control the water blast by the distance you hold the tip of the wand from the surface. On wood use 1500 psi with a 15 or 25 degree tip.
You must pressure wash all protected areas
Areas of the house that are protected from rain and sun are subject to the accumulation of pollutants and surfactant from previous latex paint. Even if the rest of the house has no discernable dust or chalk on it, you still must clean the protected areas. Avoid spraying the vents themselves, use a wire brush on them instead and then wipe them with a no rinse pre-paint cleaner.
Salt deposits
Protected areas of a house will accumulate “salts” from the atmosphere. These salts are deposited on a house when dew accumulates on the house during the night. In the morning when the dew evaporates, it leaves behind the salts. Over time the enough are deposited to interfere with paint adhesion. A lot of the intercoat peeling seen underneath overhangs are caused by salt deposits. Salts will deposited on the whole house as dew accumulates on it at night, but the salts are washed off naturally by rain and only accumulate in areas protected from the rain. Don’t assume protected areas are clean, the salts are really invisible to the eye, and surfactant may only be seen on an angle. Remove both of these by water blasting., being careful not to shoot water into the attic of the house through the overhang vents.
Chemical injectors
You can apply concentrated soaps and detergents and even kill mildew using bleach through via a chemical injector. The injectors will siphon the chemical into the water line when activated. Activating the injector is done by dropping the pressure. The chemical injector is put in line on the high pressure hose using a quick connector. Once attached , it will siphon the chemical only if the pressure drops. This is done by either changing the tip on the gun to a wide open low pressure tip, or by switching to the low pressure wand on a double wand gun. The chemical injectors will meter the chemical into the water line at a ratio of anywhere from 4 to 1 - 10 to 1 (see the label of the injector for the exact ratio). This requires the use of concentrated cleaning solutions, since they will be considerably diluted with the water at the inlet and metering process.
Applying a cleaning solution without a chemical injector
If you don’t have, or don’t want to run the chemical through with a chemical injector, you can apply it with a pump sprayer. Use a clean, new one or two gallon garden type sprayer for this. The pump sprayer is the only way to apply un-concentrated chemicals (like Jomax) that a chemical injector will dilute too much to be effective.
Loose and peeling paint (for post 1978 houses only)
Water under pressure does a good job of removing loose paint. Unlike sand blasting, (or set sand blasting), it will only remove the loose paint and leave the paint that is adhering alone. Water blasting does not therefore “strip” paint. The paint left behind will need to be followed up with a scraper and feather sanded (again read lead caution if the substrate is pre-1978) to smooth it out. You can use the Wagner Paint Eater for this very effectively.
Collecting (non hazardous) paint chips
Some contractors with put landscaping mesh material at the bottom of the house to catch paint chip debris. The landscaping material will allow water to pass through it, and yet it will collect the loose paint chips on top of it. The landscaping material can be held in place with bricks or staples or with some other fastening method. The landscaping material will catch debris that drop directly down around the house perimeter. The turbulence of pressure washers will scatter some debris beyond the landscape material which will require further clean up. You will need to do some raking with a leaf rake (or something similar) to clean up debris beyond the landscaping material. Catching the debris (with landscaping mesh) and raking will take care of much of the debris and paint chips, but not all of it. This is usually acceptable with non-hazardous debris, but it is not good enough for hazardous (lead paint) debris - see side note.
The turbo spray nozzle
The turbo nozzle is made for loose paint removal. It is a 0 degree tip that rotates in a circular manner over an area of a foot or so (depending on how far the wand is held to the surface). The turbo nozzles will remove loose paint very effectively, but they are the worst nozzles for ripping up a substrate too. Use the turbo nozzle on hard substrates. On wood substrates you may be able to use the turbo nozzle with vigilance and caution. Keep the nozzle far enough back on wood surfaces to prevent scaring the wood. If the turbo nozzle proves to be too aggressive, you can switch over to a 15 or 25 degree tip for loose paint removal.
Water blasting will leave the wood saturated with water requiring a couple of days for drying out before painting. Use a moisture meter especially if you are priming or painting with an oil based paint to determine if the wood is sufficiently dry prior to painting. The moisture reading should be (as a rule) 15% or less after correctly calibrating for the temperature and substrate type before painting.
Please read lead paint warning if your house was built before 1978
Please read lead paint warning if your house was built before 1978
Pressure Washers
Water Blasting: Pre-Paint Preparation
Using a Pressure Washer
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Extend the reach of your pressure washer with a telescoping pole
Note: Do not waterblast or pressure wash pre-1978 houses to remove loose paint. The paint chips blasted off may contain hazardous lead paint which may be widely scattered around the house making total cleanup virtually impossible. Complete cleanup is extremely important with hazardous lead paint chips - therefore pressure washer removal or waterblasting is not recommended for pre-1978 homes.
