What is Block Filler – How to Apply it Correctly?

block filler

Many of you may have heard of “block filler” but do not fully understand what it is or how it is applied.

The block filler itself is relatively simple in composition. Typically, it consists of one part cement, three parts sand, and just enough water to create a stiff patching material. 

In addition, block fillers may also contain Extender Pigments, Titanium Dioxide, and Vinyl-Acrylic Resin. But, the basic materials themselves are easy to come by, as concrete, sand, and water are quite common. 

After properly mixing in the right amounts, the block filler compound can be poured into the voids of the concrete blocks or spread onto the masonry surface to create a professional finish. When it hardens, it will last for a long time.

What Does Block Filler Do?

Now that you have used it to fill the blocks, what exactly have you accomplished? 

The answer is quite a bit since the overall strength and resilience of the concrete blocks have been enhanced. The filler will add substance to the individual concrete blocks to make them stronger. But that is not all of what they can do.

In addition, the block filler can slow down the corrosion process of materials inside the blocks. This means steel rods used to reinforce the concrete or other metal additions, which are normally subject to rusting, will be protected by the block filler. 

Even though the filler does contain moisture, it prevents the oxygen in the air from reaching the metal, which otherwise will start the corrosion process.

Where is Block Filler Generally Used?

Block filler is generally applied to cover precast cinder blocks, rough bricks, and concrete masonry units (CMU) that require a long-lasting professional finish.

The other everyday use is for concrete blocks that are part of the construction or a building structure. The blocks themselves are hollow, which makes them lighter and easier to move into position, along with having some openings for cables, wiring, and plumbing.

Once the concrete blocks are in place, the voids inside are filled using a cement-based mortar or small-sized gravel, which then have properties like poured concrete walls. The inherent strength and durability of these concrete blocks are further increased using block filler.

Applying Block Filler

How to Apply Block Filler to the Surface?

Block Filler compounds can be applied using different methods, such as brushing, rolling, spraying, or using a squeegee tool. 

The first step is to prepare the masonry surface. This means it must be cleaned and free of oil, rust, dirt, dust, or any other foreign particles – the presence of unwanted contaminants will not allow the filler to stick to the surface correctly.

Then based on the temperature, humidity, and thickness of the film required, you can use the preferred application method, which will have its own requirements and benefits.

For example, when using a roller or brush on surfaces, you should use an even, consistent pressure that ensures a uniform layer. When spraying, keep the gun at least 12 inches away from the surface and move it in a steady motion. And for squeegee applications, start at one corner of the surface and slowly move it in an even motion across the entire area.

Once you have applied the block filler to a satisfactory degree, let it dry completely to achieve a uniform texture. You can then apply the top coat of enamel, epoxy, or latex paint on the surface. 

By painting over the block filler layer, you are essentially applying another additional protective layer that seals and protects the surface from moisture. It’s especially effective on ceilings, roofs, and exterior walls that can be compromised by heavy moisture.

TIP: Most block fillers come with VOCs or volatile organic compounds. The levels are low and not considered harmful under normal exposure conditions, but it’s still safe to use safety gear like a face mask and gloves while applying the product.

Choosing the Block Filler – The Best Available Options

But to create impactful results that will stand the test of time, you must choose the Block Filler product carefully.

Some of the best Block Filler products that are ideally designed to adhere to the surface, fill and level out small cracks, create a smooth, non-porous surface for painting, and won’t shrink are:

  • Zinsser® Block Filler 2X
  • Valspar® Professional Block Filler
  • Sherwin-Williams Pro Industrial Heavy Duty Block Filler

Besides the above, Sherwin-Williams CONFLEX and PrepRite Block Filler are also highly recommended products for their superior adhesion, excellent hold, and high durability.

The Bottom Line

The block filler is typically a filler material for concrete, bricks, stucco, cinder block garden wall, and the like.

It’s the perfect choice for any interior or exterior masonry finish, offering superior performance and protection when applied beneath epoxy, alkyd, and latex top coats.

This means no voids, cracks, pinholes, or similar imperfections are present once the block filler has been properly used – it’s much similar in function to applying caulk or putty to drywalls before they are painted.

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