Advice on sealing polished porcelain

[t5_nel]

So my polished porcelain tiles arrived yesterday and I have checked a few random samples out.

All looks good.

There is a very clear instruction that the tiles must be sealed after laying - as stared by all on here.

I have a couple of questions...

1)Why do you need to seal before grouting? Is this just to avoid the risk of grout staining the tile? If you do it after grouting is it really too late or better than nothing.

2)Does the sealant in anyway change the look of the tiles - obviously they are very flat and highly reflective at the moment. I am converned that this will change if the sealant leaves a (varnish like) film on top of the tiles as this will contain marks from the application.

3)What products do you recommend (I favour something that is easy to put on and is 'ready' quickly)What is the coverage like - does the guidance on the tin or bottle tend to be accurate or hopelessly optimistic

Thanks
Tim