I would like to mention a few things about re grouting mortar joints.
Make sure you work this into the joint by pressing on your grout trowel (known as a Slicker in the trade) 1st pic below, or what ever you decide to use for this and don't be bashful with the amount you use. Leave a bit more on and you need after you have worked it in and let sit for 1-2 hours ( if useing mortar) then remove access amount and lightly go over it with a small broom or brush. Here are a couple of tools we use in brick laying. One is for regular square joints and the other is for rounded joints. Hope this helps...