How do I clean new slate tiles?

[MarkandCath]

We have just laid 30 sqm of slate tiles and they have white marks/stains on the edges. All the marks are on what was the bottoms when they were stacked on the pallet. I assumed they were just water marks from standing outside and would just wipe off. We have tried a stiff nylon brush and a stone floor cleaner (mild acid) but the marks are still there.

We have lightly sanded an off cut and it takes it straight off, so the marks are only on the surface. So, without sanding, how do we clean them up before we seal them? Thanks.

 

[foxhole]

remove stains by mixing water and hydrogen peroxide (in equal measure) and spraying it onto the stained area or a slate cleaner.

 

[MarkandCath]

remove stains by mixing water and hydrogen peroxide (in equal measure) and spraying it onto the stained area or a slate cleaner.

Thanks. We have tried the standard slate cleaner but it doesn't seem to touch it. I will try and source some hydrogen peroxide.