Flex Seal Liquid for Basement Waterproofing

“Put this on a basement wall with a hairline crack from top to bottom and mud was washing in at the bottom..
We hoped it would fix the problem since a relative had the same problem and it cost them $12,000 to fix their basement.

This week it rained for three days, without this applied to wall we would always have water and mud run across the floor. we have checked it several times and ‘NOTHING’ We are so excited about this product. I was skeptical but it seems to be hold back the leak. I wish we had heard about it a year ago...”

– Alma Atckinson, Amazon Customer

Basements have a bad reputation for storing moisture and mildew. Since they’re located below ground, water from poor exterior drainage, heavy rainfall, or a high-water table can find its way inside through porous concrete walls.

As a homeowner that is always up for the challenge to fix and prevent any potential problems under my own roof, knowing how to seal basement walls was something I was determined to master. After all, if basement water intrusion is ignored for a long period of time, it can create extensive structural damage and mold problems. To avoid that from becoming my reality, I turned to Flex Seal to waterproof my basement walls.

Read on as I explain how I sealed my basement walls using Flex Shot and Flex Seal Liquid MAX.

Before Getting Started

The first step for me wasn’t to head straight to the basement, it was to properly inspect the entire inside and outside of my home. This step is critical for anyone looking to seal their basement walls.

If the walls in your basement are actively leaking, identify the problem at the source. Just sealing your basement walls without finding the root cause is not recommended. You should determine whether you need to add gutters at your roofline, install proper drainage measures, or complete a separate exterior repair to prevent water from entering your home.


● Wire Brush

● Efflorescence Remover

● Utility Brush

● Shop Vacuum

● Dehumidifier/Fan

● Disposable Gloves

● Blue Painter’s Tape

● Drop Cloth/Plastic Sheeting

Flex Shot

Flex Seal Liquid MAX

● Paint Brush

● Plastic Paint Container

● Paint Roller (¾” or 1-¼” depending on surface)

● Paint Tray

Steps for Sealing Basement Walls

For this project, I’m going to be using Flex Shot and Flex Seal Liquid MAX. These two products work well together to provide a superior water shield. Below is a step-by-step guide on how I used Flex Seal products for waterproofing my basement walls.

1. Prepare the Area

First, clean the basement walls to ensure an optimal surface for the Flex Seal products to bond to. Use a wire brush to remove any loose paint and dirt. Be sure to wear a face mask when using the wire brush so you do not breathe in any concrete dust particles.

If your concrete walls are showing any signs of efflorescence, (a crystalline deposit of salts that can form when water is present in or on building surfaces) you can use an efflorescence remover to clean up those locations. Mix the remover with water according to the package instructions and apply with a stiff utility brush. After applying, rinse the area with water a few times.

Clean up any dirt, concrete, or paint dust that was removed from your walls with a shop vacuum and set up a dehumidifier and/or fan to aid in the drying process. Allow your walls to dry completely before applying Flex Seal products.

2. Apply Flex Shot

Before you begin, make sure to read all directions and set-up a fan to circulate fresh air into your work area for proper ventilation.

Start by applying Flex Shot into any concrete cracks, joints, and holes. Flex Shot can be applied directly from the tube by pressing the nozzle - no caulking gun needed.

Flexpert Tip: Put on a disposable glove and use your finger to push the Flex Shot into the concrete opening. This will ensure the product completely fills any gaps and provides a complete seal.

After all the holes, cracks, and joints in your concrete walls are fully sealed with Flex Shot, let the product dry for 24 hours.

3. Apply Flex Seal Liquid

Once the Flex Shot is cured, it’s time for Flex Seal Liquid MAX. This is the same formula as Flex Seal Liquid, just in a bigger, 2.5-gallon size, which is perfect for large projects. Flex Seal Liquid is thicker than paint and dries into a strong, rubberized coating. It can also help seal out air, water, and moisture and is chemical and mildew resistant - making it a great choice for your basement walls.

Start by taping off the walls you will be waterproofing with blue painter’s tape.

You may also want to place a drop cloth or plastic sheeting on your floors to avoid spilling any excess product where you do not want it.

Now it’s time to take it to the MAX! Open your container of Flex Seal Liquid MAX and stir the contents before using.

For ease of application, you can transfer some of the Flex Seal Liquid MAX to a smaller container.

Using a small paint brush, begin applying the product to your basement walls - focusing on corners and edges before you transition to using a roller.

Be sure to work your Flex Seal Liquid MAX into all divots, pinholes, and joints for a complete application on all surfaces. Applying the product slowly and with care will ensure adequate coverage.

Once you have painted the corners and edges with your brush, it’s time to set-up your paint roller. The roller size you use will depend on the surface of your walls. For a standard, rough concrete surface, use a ¾” nap roller. For an even rougher concrete surface, opt for a 1-¼” nap roller.

Flexpert Tip: Flex Seal products can be washed off paint brushes and rollers by soaking them in mineral spirits. You can also opt to purchase enough brushes and rollers for two coats of the product and then dispose of the tools afterwards.

Flex Seal Liquid MAX is a self-leveling product, so as you’re putting it on a surface, it may look like a rougher texture, but it will begin to smooth out. Make sure to apply the product slowly, pushing it in as you go as opposed to rolling it so that it enters all the cracks and divots to create a level, sealed surface.

Review your painted surface and use a paint brush to fill in any remaining holes or cracks.

Allow 24-48 hours for your first coat to dry. Two or more coats are recommended for a complete, moisture-resistant seal. Although you cannot paint over Flex Seal Liquid MAX, you may apply as many coats as you’d like.

Once the final coat is dry, it’s time to bask in the glory of your freshly sealed basement!

By Matt Lanteigne

Matt "The Fixer" is a first-time homeowner with over a decade of experience in construction. He uses that experience to work on his "fixer upper" and take on all kinds of projects in between. His goal is to capture his work and knowledge on video with the hope that it will teach, inspire, and motivate others to tackle their own home repairs and DIY projects!