Too late to help you, but the chap who did our beigy stone type tiles suggested using a pale primrose - coloured grout to avoid this problem. it has worked really well many years on but of course it depends on the colour of the tiles.
I always open the window in the bathroom for a few minutes after a shower and I also leave the bathroom door open. This helps to reduce moisture building up and grout from blackening. I put the extractor fan on too but I’m not convinced that it does much! There’s a bleach spray for bathrooms made by Dettol that instantly removes mould on tiles, grouting and around windows if you use it as soon as you see any blackening. A miracle cure! It’s worth a try on your grouting.
I have both the Karcher window vac and a squeegee and the latter works far better. Our bathroom tiles have a very pale blue grout and that keeps them looking ‘white’ even when aging.
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
I did try rubbing at the Ronseal with a dark purple paper cloth and -yes - the whitening does come off, weeks after application.
I guess it's no kind of "grout" despite the label. Is does not at all "do what it says on the tin". It's more like white paint to cover up the problems.
"Ronseal One Coat Grout Pen Brilliant White"
I suppose it could be said that the name is a bit ambiguous. It's a pen to put on grout, not to regrout.
Ah, thanks for the test, Fire. It seems it is a touch-up exercise rather than a solution. What is needed is something like nail varnish to kind of seal and colour over at the same time. Presumably there is a good reason why such a product has not been invented. 🤔
Yes! That was my original idea, but I was persuaded out of it on the grounds of noise, mess, and labour costs. The whitener seemed like a reasonable alternative. You live and learn. It'll do for now while I accumulate another list of jobs and it can be tacked onto that.
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I'll try to be very careful not to rub the surface when I'm drying it, and hopefully not too much will come off.