How to Clean Exterior Walls Before Painting?

Like most paint jobs, the key to a long-lasting exterior paint job is proper preparation. Before you can start slapping new paint on your house’s exterior walls, you’ve got to clean them thoroughly.

Exterior paint can last 15 years or more – providing you properly clean and prep before applying paint. If you skip this step and paint over dirty surfaces, you’ll end up with a poor-looking result that won’t last.

So, how do you clean the exterior walls and siding before painting? Should you use a pressure washer? Or some type of cleaning chemical?

I’ll break down everything you need to know to clean your exterior walls before painting.

Why do you need to clean exterior walls before painting?

You might be tempted to skip cleaning exterior walls before cleaning, after all, it’s a dirty job, and won’t the new paint cover up the old dirt and grime?

Well, yes it will cover it up temporarily, but 6 months from now you’ll be dealing with chipping, cracking paint, and be back to square one.

Removing dirt, grime, mold, and mildew from your house’s siding will result in a clean surface for the new paint to adhere to. Paint adheres best to clean, dry, and sanded surfaces, so you’ll want to ensure the siding dries for several days after cleaning.

Also, keep in mind the best time to clean exterior walls is during the warmer summer months. You want the siding to dry quickly, and avoid moisture getting trapped between the siding. During winter, you can run into problems with water freezing or not evaporating quickly enough.

How to wash a house before painting?

Ok, so how should you wash the exterior of your house before painting?

There are essentially two different ways to do this: scrub and wash it by hand, or by using a pressure washer. We’ll take a look at both methods in detail, and explain the pros and cons of both.

Method 1: Hand Cleaning

Hand cleaning is exactly what it sounds like. Scrubbing and washing the exterior with some good old fashioned elbow grease.

scrubbing house exterior by hand
A little old fashioned elbow grease is the way to a well-cleaned exterior.

Hand cleaning is more labor-intensive than the alternative, but it has less potential to damage the house and results in a better finished product. You’ll need to get up on a ladder to do this properly, so make sure you’ve got a quality ladder, and are comfortable using it safely.

Tools & Materials Needed

  • Ladder
  • Scrub brush
  • Rags
  • Garden hose with spray nozzle
  • Mold/Mildew cleaner like 30 Seconds

Below is a step-by-step guide to cleaning a house’s exterior. It’s best to work in sections, rather than doing the entire house at once.

Step 1: Assess Exterior Condition

The first thing to do is to assess the condition of the exterior. If your siding hasn’t been cleaned or painted in decades, you’ll need to do a more thorough cleaning than a recently painted house. Check for chipped and cracking paint, cracks or gaps in the siding, and mildew/mold/algae. Pay close attention to the north side of the house, as the shade and damp provide ideal conditions for mildew to grow.

If your paint is old and chipping, be careful when hosing it down. This old chipped paint will need to be removed before repainting, but chipped and cracking paint can be a sign of more serious damage beneath. Also, take a look at the condition of the paint around the window frames and trim.

Step 2: Scrub/Brush Away Dirt

Use a large brush to remove any loose dirt and debris attached to the house siding. There’s no need to go crazy here, as you’re going to wash the exterior walls afterward.

After you’ve brushed away any large pieces of dirt, use the hose to spray down the siding. Take care not to spray upwards into gaps in the siding, as this can cause moisture to collect inside the walls.

Step 3: Apply Chemical Cleaner

Spray down the entire wall with a mold/mildew cleaner for 30 seconds. These bleach-based cleaners are specifically formulated for this task, and are incredibly effective and removing dirt, grime, and mildew.

spraying down house exterior with 30 seconds cleaner

One thing to keep in mind when using these cleaners is to wear old clothes you don’t mind ruining. The cleaner is bound to get on your clothes, so don’t wear anything you want to keep pristine.

Step 4: Rinse Clean

Let the spray soak into the exterior as per the manufacturer’s directions. Then spray down the siding and watch the dirt and grime melt away!

washing away mildew with garden hose
Watching the grime and mildew melt away is about as satisfying as it gets!

After this process is completed, you might find stubborn spots that weren’t completely removed in the first pass. Spray these areas again, and work the cleaner into the siding with a rag. Then rinse clean with the hose.

Finish the exterior off with a final hose down, and then let it dry for at least 48 hours before applying paint.

Method 2: Pressure Washer

Tools & Materials:

Renting or buying a pressure washer to clean exterior walls can make the task less labor-intensive, but you’ll need to take care not to cause accidental damage to your walls.

A pressure washer can put out 2500 to 3500+ PSI of pressure, which is far more than the typical 40 to 80 PSI found in a garden hose. This massive amount of water pressure makes pressure washers fantastic at cleaning driveways, sidewalks, fences, and vehicles, but a little overkill for houses with wood siding.

If your house has vinyl or brick siding, then damage from the pressure washer is less of a concern. If the siding is wood, you’ll need to be cautious about spraying in-between gaps and around windows/vents.

When pressure washing, you’ll want to wear solid eye-protection, avoid making contact with the jet, and never spray from a ladder. Also, if your house was built before 1978, make sure to check for lead paint before starting.

To pressure wash a house before painting:

  1. Cover up shrubs, gardens, and other sensitive vegetation before starting. Close and seal any windows, doors, and light fixtures.
  2. If you’ve got mildew to deal with, use a cleaner with mildewcide to thoroughly soak the siding before pressure washing. This will remove most of the mildew, thus requiring less pressure later on.
  3. Power up the pressure washer, and starting from the top of the house work your way downwards. If you’ve got a two-story house, a wand extension will be necessary. You may also want to use a moveable scaffold if you’ve got a lot of square footage to clean.
  4. Angle the sprayer downwards as you work your way around the house. This accomplishes two things. First, it prevents water from getting underneath the siding and inside the walls, and secondly, it’s easier to keep the nozzle under control.

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