Dirty joints

Dirt naturally accumulates in the joints, turning them black and unhygienic. To clean the joints in stone surfaces, we recommend using a non-aggressive alkaline product like PS87 PRO diluted 1:5-1:10 (1 litre of product per 5-10 litres of hot or lukewarm water), using a brush to remove the dirt. Rinse abundantly. 
Once the surface is dry and clean, you can finish off the stain protection treatment with the product most suited to the type of natural stone in question.