Maybe I'm giving up prematurely but I've tried several tips. Plus some of our grout is falling apart.
Thus I've taken on the uneviable project of removing all the grout in our shower tile floor.
What's the best grout removal tool for this job?
An oscillating multi tool and a rectangular grout removal tool will help you remove grout lines without losing your cool (or hair, unless you're already bald-sorry).
Here are some tips
-Apply two layers of duct tape along surfaces you don't want to scratch with the tools
-Use a grout attachment with the oscillating tool
-Measure the width of your grout lines to determine the correct multi tool attachment size
-Remove grout down to 1/2 the thickness of the adjacent tile (e.g. if your tile is 1/4 inch thick take out at least 1/8 of an inch of old grout)
-Vacuum the are grout lines frequently to make sure you're not penetrating the shower pan or substrate the tiles are attached to
You'll discover more pictures, tips, and resources by clicking on this link to my blog post http://www.homerepairtutor.com/best-grout-removal-tools-for-tile-shower-floors/
Have a great day!