I'm just getting ready to lay down the grout after successfully tiling our master bathroom. I've been told in no uncertain terms that I must remove the spacers between tiles (or else grouting over them can result in cracking of the grout). That makes sense — however, I'm also reading on the box that I should "clean the grout joints". They show a tool that appears to be scraping excess mortar from the joints.
Do I need to be worried about scraping down the hardened mortar so that the joints can be completely filled with grout or are they talking about simply removing any loose mortar? Although none of my joints have mortar that rises above the grout, many of the tiles have some sections where the mortar is higher than others. I'm reading elsewhere that it's ideal to get rid of all excess mortar between tiles before it hardens (whoops).
How important is this?
Update:
As of September, 2011, it's been 9 months since I completed this job and things have been solid. I scraped only the areas that were high (close to or slightly above the tile). From this purely anecdotal evidence, it seems logical to conclude that it isn't necessary to be too much of a purist about clearing out all mortar between tiles before laying down your grout.
Update 2:
As of April, 2012 I've found only one tile that may have a slight issue. On one of the center tiles that gets a lot of traffic, I must not have put down a perfectly smooth layer underneath because I can hear an occasional slight shift. The joints along the edge still look great though.
Best Answer
I wouldn't be too concerned unless there's mortar that is raised above the level of the grout - which you've already said there isn't.
Scraping the higher sections to give a reasonable depth of grout would be as far as I'd go.
A lot of adhesives these days are sold as tile cement and grout, though they do tend to be coarser than "pure" grout, so they are basically the same material. However, a pure grout does give a smoother, "cleaner" line.
Floor tile grout also tends to be coarser - I suspect to be able to cope with the higher degree of wear and tear it will receive compared to wall tile grout.