cleaning cast cement stone

[plasticpigeon]

Hello. I had some cast stone window sills made as they were cheaper than real stone. They looked nice to start with, but they get dirty and mark really easily. Even mud from boots when I stood on one seems difficult to remove completely. Worse still tape residue from putting up sheeting when painting seems impossible to remove, or more accurately I believe I have removed the residue with solvent, but the sill is very discoloured underneath, and I got a few paints spots on the sills too. I have tried a couple of strippers on the paint and tape discolouration and nitromoors doesn't touch the paint, and a sample from stonehealth for stripping acrylic paint (it was a waterbased primer that I spilled) didn't remove the paint, but did make the sill a lighter color in that vicinity which looks very ugly. I am now trying a patch with peelaway 7. I would be happy to lighten the whole sill if I could get it a consistent colour, as I think they may have gone a little darker with natural weathering even in the couple of months they have been on so should weather down again. Is there anything else I could try please. I am now thinking I might limewash them but this would be really disappointing since I bothered to get them colour matched. I will avoid cast stone in the future, as if this was real stone it could be cleaned more easily, or even sanded slightly to remove marks. cast stone seems porous but really hard, so can't really be sanded or worked with a tool at all.

 

[88v8]

That's a shame after all your efforts.
Perhaps you could go the other way, spray it with sour milk and encourage lichen to grow on it.

Ivor

 

[masochists-r-us]

You could try dilute hydrogen peroxide, but patch test first.

 

[plasticpigeon]

I just phoned the manufacturers and they said that id=f I thoroughy wet the sills first I can use brick acid which will erode the very upper surface and even the colour out!!!! Might try that in a very inconspicuous place. The hair bleach sounds like one to try too.

 

[xanthe]

You've probably seen this, but just in case...

http://www.buildingconservation.com/articles/poultices/poultice.htm

 

[mr-squirrel]

What the manufacturers say sounds about right - they probably get asked all the time.

Straight from the bottle brick acid it is normally quite powerful so I would dilute it down first. It is very nasty stuff - use eye protection!
You can always go stronger if required (or leave it longer) to get the right effect. Not so easy to go the other way!

Saturating with water will kill the suction in the stone and stop it sucking in the acid too deep (that can be quite damaging). You should really soak the stone until it won't take in any more water, wipe down any water standing on the surface, then do the acid - watching it like a hawk. Maybe a little scrubbing with a plastic brush if required.

When it has had the desired effect wash down with plenty of water to neutralise the acid. Ideally do this with a hose (which won't run out, unlike a bucket) as you don't want the acid getting absorbed into masonry underneath the sill as it runs down! Actually I'd soak the wall below it beforehand to lessen the chance of any acid running down getting absorbed and leaving streaks. Same goes for slabs or anything cement/lime based nearby that might get some acid when washing down. And watch where you step - you don't want to tread acid across any executive concrete paving you might have

Have the hose set up beforehand so you can halt it's progress as soon as you are happy with the results. You can always re-apply the acid again or change the strength if you feel it is heading in the right direction and needs more. I'd do lots of standing back and checking, definitely test it on an unseen area first.

Any solvent you have used on the paint spots will have inevitably thinned the paint and caused it to be absorbed deeper under the surface, leaving a stain. So although the paint spots might have gone, you are left with a stain. Bear in mind that the brick acid may not work correctly on the stained areas.

You could sand the surface with an abrasive diamond hand pad (a few grades available and normally used for finishing tiles and slate etc, look on eBay) but be careful not to go too deep. That will abrade even the hardest concrete/stone no problem.

 

[plasticpigeon]

Yes, I tried the brick acid on a well soaked sill, and it very slowly eroded the surface. Not all the stains went but almost all, and the colour is much more even, so this is the way to go on cast stone it seems.