Dark Marks After Trowelling
Dark marks on the microcement surface are caused by the steel of the trowel dragging over the surface with a lot of pressure.
Here’s an explanation of what causes them, whether they matter, and how to minimise them.
Why Do I have Dark Marks in the Surface of my Microcement?
This happens when the surface has been trowelled a lot of times with a lot of pressure on the trowel. As the steel of the trowel is rubbed over the surface, it causes the surface to blacken in areas.
The areas that blacken more tend to be raised slightly more than the rest of the surface. So the blackening might highlight trowel marks left in the previous coat or imperfections in the substrate.
Are dark marks a problem?
Not at all, in fact, some installers do this on purpose to achieve more visual texture, the technique is called burnishing.
Burnishing doesn’t effect the performance of the products. But it can look too busy or like dirty marks in lighter colours.
How do I get rid of dark marks?
To resolve, sand the area flat and apply another top coat, this time not putting too much pressure on the trowel and not going back over it too many times until desired finish is achieved.
You may want to try a plastic trowel or another product like Microne Continuous which will naturally burnish less than Stucco Effect.
You will also find that the surface will be more uniform if you leave the first top coat to dry fully before applying the second top coat.
What if I’ve already sealed the surface?
Once the surface has been sealed, the only way to change the appearance is to recoat the surface with microcement. You will need to sand the sealer, to give it a key, prime then reapply top coats & sealer.
See our guide to Recoating Microcement after Sealers for full details of what to do if the surface has already been sealed.