Surface Preparation

• The surface must be free of moss, moisture and dirt for new grout to bond to old brick patio grout joints. Loose old mortar also must be removed. Pressure washing is a handy option for removing debris. Use a pH-neutral soap to remove surface contaminants.
  
  

Applying New Mortar

• Medium thick mortar/grout adheres to the brick grout line best. Consider using an acrylic admix to strengthen the grout. A grout bag (sold at masonry supply stores) works like a pastry bag, allowing the do-it-yourselfer to delicately squeeze the grout into each joint and avoid making a huge mess.
  
  

Prevention

• Loose, cracking or missing grout is a moisture-related issue. If you want the mortar in an old brick patio to last a long time, seal it after the first grout application or re-application and seal it twice a year after that. If the elements don't have a chance to permeate the grout in the future, chances are the job will last much longer.