Answer: This issue of blotchy and inconsistent color and gloss created by a sealer is not that uncommon. There are multiple factors that can create this appearance, but the two most common are application — how the sealer was applied — and surface — what the sealer is being applied to in regard to density and porosity.
The culprits of blotchy concrete after sealing
When you find yourself staring at discolored concrete that has already been sealed, there are usually three potential sources of blame: The sealer was applied poorly. Not enough sealer was applied. The concrete itself had varying absorption rates.
When you apply a sealant to your pavers, it gives them a clean, wet look with a glossy finish, depending on the finish of the sealant you use. It will not alter the color of the paver but will instead highlight the hue of the material and enhance the color of the pavers.
Blotchy and Dark Concrete: Blotchy concrete can result from over application. In order to prevent over application, apply sealers in thin, even coats. If over application does occur, use Xylene to correct the problem.
When the sealer is applied too heavily, the air displaced through the surface can't escape, and it forms a bubble in the sealer surface. Bubbling of a sealer can also occur if application is done in hot weather, or if the concrete is in direct sun.
Sealers, particularly solvent-based sealers, will darken concrete. If unevenly applied, it will create light areas where there is less sealer, and darker areas where there's more. If applied by sprayer, it may appear like “stripes”. Uneven roller application may leave darkened roller marks.
Film-forming concrete sealers will darken concrete and leave a glossy shine to some extent, giving the concrete a “wet look." The concrete will lighten back to its unsealed color over time, as the sealer wears off or is removed.
Vinegar or baking soda: These natural cleaners work well to clean cement patios with stubborn charcoal or rust spots. Mix equal parts vinegar and water, scrub the patio with the mixture, then give it a good rinse. If stains don't dissolve, try undiluted vinegar.
Factors influencing the discoloration are calcium chloride mixtures, cement alkalis, hard surfaces, inadequate or insufficient curing, wet subsurface, varying proportions of aqueous cement on the surface, and changes in the concrete mixture. Discoloration from these causes appears very soon after concreting.
A strong enough power washer will blast most types of sealer off of concrete with ease. The trick is to make sure to pressure wash the entire surface area. For more sturdy sealers, it may take a few passes with the pressure washer to get it completely removed.
However, “too much of a good thing” can come into play, and you should be wary of over-applying paver sealer by reapplying too often, which could create a haze.
Simply put, black stains on flagstones are caused by certain types of mold and mildew. These fungi find their way onto the stones either from their spores landing on the stones or from simply extending onto the stones from patches of mold or mildew right beside them—a pretty common occurrence for garden pavers.
Using too much sealant could deposit a hazy coating on the pavers, making them look faded or discolored. Sealing isn't a one-time deal. Once you start sealing pavers, you'll need to continue re-sealing every 1-2 years. There are no guarantees, even if you use the highest quality sealant.
Prepare a mild, non-acidic cleaning solution.
If you're going the DIY route, all you need to do is grab a large bucket and combine warm water with a mild degreasing dish detergent. Stir the solution well, then evenly distribute the solution over the pavers. Do not use acid-based cleaners as this can damage the seal.
Applying two coats of sealer to a porous paver may look good for a short period of time but the paver will continue to absorb the sealer and within a short time will leave the paver looking dull and often appears to be unsealed.
Lichen are white, brown, yellow and black (i.e. "black spot") biological growths that live on slate and concrete tiled roofs, as well as on concrete paths, monoblock drives, brickwork, and occasionally on wall render. Lichen can be very tough to remove and pressure washing alone is ineffective.
Apply a cleaning product such as SunSpot, a concrete degreaser, Natural Orange (or other citrus cleaner), Simple Green, or automotive brake cleaner to the affected area. Allow the cleaner to sit on the surface for a few minutes. Scrub the spot with a brush with stiff bristles.
Antiquing Concrete Stain is a great option to blend the colors on more porous floors. Concrete dye can also be used to cover imperfections and accent the floor as needed. Topical stains can disguise the tile ghosting still visible after grinding.
If you notice dark spots on concrete after pressure washing, it's likely that the surface is damaged and that the top layer is washed off. This type of damage is prevalent in slabs that are topped with a screed with a different color from the base concrete.
Before applying a new sealer, it's very important that you make sure the old one is completely gone. This is because any old sealer left on the surface of your concrete can impede the effectiveness of the newly applied sealant, preventing proper adhesion to the concrete surface.
Film-forming concrete sealers will darken concrete and leave a glossy shine to some extent, giving the concrete a “wet look.” The concrete will lighten back to its unsealed color over time, as the sealer wears off or is removed.
A matt stone sealer will not change the colour or appearance of a stone floor. A polished stone sealer will add a slight sheen to the surface of most stone tiles.
Apply the first coat, waiting at least two hours before applying the second. Check your sealant can for specific drying times. Apply two thin coats to ensure a smooth and even finish. If you're sealing an interior concrete floor, make sure that you have plenty of ventilation.