I need to remove a lot of grout between tiles and regrout with
waterproof grout. Does anyone know of a cheap rotary tool that can do
this. Those Fien Multimaster tools look like they can do the job with a
frighteningly high price. There must be a chep DIY effort around that
can do a similar job. Those rotozip type of tools look like they could
wander and cut into the tile, so I am ruling those out.

TIA.