It would be appreciated if someone could recommend a superior, outside grout for a slate walk. Also, former grout repairs has resulted in various shades of white, which detracts from the appearance of the walk. Is there some way that aged grout could be made to look more uniform in color.
In the past the best results that I have received in this situation is to use a mortar mixed with a latex additive and colorant if chosen, First remove as much of the old so you can use more of the new. Grout stains easily and cleans easily. If the only problem are the stains, You may entertain the option of cleaning it first and letting it dry. However stains will always be an issue with grout and grout sealers are not and outdoor option. A good satin Poly will do the job and give the protection that is needed.
This goes back to restoring old buildings but you can purchase ground stone to mix in with the morter. It's a lot like being an artist and you will mix quite a few small batches and let them cure before finding the right one or combination but once the color is right it seldom fades, even in the Florida sun.
Last year I revisited a job that we did in Ft. Laud. in the late 60's. With proper mantainence it would still be perfect, as it is, it's still pretty d**med good.