OK, first let’s get the obvious out of the way; Is the rust coming from a metal fitting of any kind, like the shower outlet or slide rail for example? – If so then you have to take that down and treat/replace that.

However, some slate can create rust all by itself; within the slate there may well be some form of iron-bearing mineral, we see this a lot in the autumn and multi-coloured slate, as well as in granites and marbles, limestones and other rocks. All that is needed is some moisture and oxygen to oxidize the iron and you make rust. So the question is, is there some moisture getting into the slate? This could be through an ineffective seal (not so likely with 4 good coats as you say, although sealers are not waterproofers, they are stain resisters). Maybe there is another reason, something leaking behind and finding its way to that part of the slate, or fine cracks in the grouting, fine gaps opening up between the grout and the edge of the slate (over time with movement, no sealer will prevent this).

So have a good look, see if you can identify the source of the moisture – anything obvious – fix that first. Also bear in mind that impregnating sealers are not coatings, they will not put a barrier between the surface of the slate and the water (they are inside the slate as a stain barrier). So any iron in the actual surface will be exposed to the shower water and the problem could be that simple.

To remove it, use a mild acidic solution, preferably one based on phosphoric acid (they use this in the motor trade to remove rust from metal) so Aqua Mix Phosphoric Acid Cleaner or Substitute or for a larger problem, Aqua Mix Extreme is excellent. If it is coming from the surface of the slate then it will re-occur – just keep cleaning it until it stops, you may consider a coating sealer to add a bit of a barrier a the surface but even this may not prevent it 100%. If the problem is from some underlying leak then you have to get that fixed and then it should not reoccur.

Hope that helps

Ian

It is hard to advise without seeing the slate. Some slates do have other minerals in them, including sources of iron, so it is not impossible for slate to have rusty veins within it. If this is the case it may be impossible to remove as it is part of the stone. However, if it is a rusty deposit (perhaps the slate was stored somewhere for a while and something metal was lying against it for example) then it should be removable.

Acidic cleaners are frequently used to remove rust, you could try Extreme by Aqua Mix – just pre-wet the stone, pour some neat Extreme onto the stain, leave it to dwell for a minute and scrub. Remove and rinse. – Do you have any pictures you could send me? – Might give me a better idea of the problem.

Ian