I saw a limestone staircase today. The customer wants all the joints made invisible (see attachment). The gaps are between the treads & risers, and between the outer/inner strings and treads. We are only talking about gaps of about 1mm, but they are quite visible. I was thinking of using a limestone wall grout and a grout gun, but have since gone off the idea of the gun. I think the grout and a finger should do the trick! However, the outer & inner strings are made of wood (which is painted). Do you think the flex grout would be flexible enough? or do you think I should be applying a very small bead of transparent silicone? Also, the treads need to be chamfered (as they have quite a sharp cut edge). Could I use wet & dry sandpaper or would a mechanical method be more appropriate? Cheers Sean