Posted: Thu Aug 16, 2018 10:05am
If I may add me advice for your consideration.
Firstly, I endorse Jim's advice, you MUST take out the old grout. YES... this is hard work, messy and time-consuming, but like with everything if you get a good base to work with then you are already ahead overall!
So, to make your life easy I would invest in an oscillating tool. I bought an expensive version (I use it a lot!) but I know they are available from around 50€, maybe even less. I have even seen them in non-food sections in the likes of Aldi and Lidl. Obviously, the expense is relative to the amount out grout you need to remove. I would further recommend you stay away from using a Dremmel type device, the small size of the attachments can lead to running off the grout and across the tiles, leaving a snail trail of a scratch across the ceramic! (Just my opinion)
Once you have your grout out, vacuum before you wipe down with a damp cloth. This will actually make your life easier in the cleanup process! Back to having a good base to work with, ready for your re-grout.
You haven't mentioned if your tiles are floor or wall tiles, in the bathroom or not, so I would also make sure you buy the appropriate grout for your room/use type.
For the application and finishing of the grout, I would suggest you spend a few mins (or more if you have time) watching youtube videos of how to do it. It really is NOT difficult, it is, however, quite a time-consuming task.
I hope that all helps and please, do NOT just use a grout paint stick! Yes it will take you minutes, but it will look awful in a matter of a days/weeks (depending again on where the tiles are)
The final point Jim makes about colour is important too. People cannot believe it when I advise NOT to use white tile grout on the floor, even for in between white tiles (there is a caveat to that comment depending upon tile and grout size!) The moment a floor, which has white grout, is mopped... it has started the staining process, it will very soon look off white and dirty.
Always work safely, please at least wear a dust mask if you are using an oscillating tool to remove the existing grout... there is likely to be LOTS of dust!
Good luck with your project and maybe take before and after photos and post to encourage others to undertake the task!