Regarding the chalk which is becoming harder and harder, I had the chemists of a renowned paint manufacturing company designed a blackboard paint which features an extreme surface hardness.

 Hue mossy-green - non reflecting and anti-glare.
I roll and spray, using the Fine Coat method. Fine Coat is a low-pressure spraying technology working at a high air volume and a low air pressure.
As opposed to the traditional coating, this method provides a high efficiency and an optimum surface quality while the environment is preserved.

Recoating: 

 Worn surfaces are ground, smaller cracks are filled with a hair spatula, larger holes are filled with a 2K spatula, the blackboard is reground, primed and then recoated.

The chalk board is ground and newly sealed.

 

Glass blackboard cleaning: 

Example: 

 

   Improperly applied blackboard paint is pickled

   Aluminum edges are decalcified and polished

   The glass surface is cleaned, using a grinding procedure

Service - Safety check