Acid bath anyone?

[jess marley]

I'm trying to remove the layers of paint on our exterior slate hangings. Although laborious, I can get the paint off with kling-strip (although sand blasting with a fine sand might work better)..thing is, even when all the paint is gone, roughly 50% of the slates are left with patches of what appear to be lime-scale deposits where the mortar from the wall behind has leached out over the surface.
Could any of the chemists out there tell me if it would be possible to remove these with an acid of some kind? I used to use nitric for etching and could probably track some down, but don't know if this would be better than hydrochloric or sulphuric...or would work at all in fact.
Thanks.

 

[andrew gayton]

ummmmm acid might not to the slate much good! can you not turn the slates around?

 

[Nemesis]

Or leave them to see what the weather does?

 

[Gervase]

You could try a proprietary lime-scale remover from the local hardware shop, or a little brick acid on a test piece. I got rid of the lime stains on our floor with some very fine wire wool and green Scotchbrite pads, but if the slate is very soft that may not be such a good idea.

 

[andrew gayton]

lime stains - use white wine vinegar and lemon juice 50/50 mix.

brick acid is basically hydrochloride acid....... i think.

or

try a weak caustic soda (£2 a bottle from boots.... what furniture strippers use) with a good wash down afterwards??

 

[Nemesis]

My experience of light lime staining externally is that it does wear off over time.

 

[jess marley]

Nope, can't turn them around as they have a bevelled edge to shed water.
All propriatry lime-scale removers merely fizz weakly and die.Same with caustic soda, lemon juice etc.
I really need something I can make bloody great vats of and immerse the slates in, as there are really rather a lot, about 120 sq.metres.
Will check out brick acid....and sand-blasting.

 

[Nemesis]

Sadnd balsting I wouldn't even consider - it can do dreadful damage in the longer term.

 

[jess marley]

Surely there's no harm in trying a corner to see if it works.

 

[Nemesis]

Sure - but in the long term it can cause accelerated erosion.

Why not simply leave it alone for a year or two? It's bound to look a tad raw at the moment.

 

[biffvernon]

There's slate and there's slate but the sort that came from Snowdonia should not be affected by the sort of acid used for brick cleaning. Probably.

 

[Penners]

white wine vinegar and lemon juice 50/50 mix.

And if it doesn't work you can always try it in your gin. :x

 

[Flyfisher]

All propriatry lime-scale removers merely fizz weakly and die.Same with caustic soda, lemon juice etc.

If it's fizzing then it's probably working; maybe you've got such a thickness built-up that it just needs longer?

The stuff I've used for the kettle tends to smell of vinegar. Perhaps a more concentrated solution of acetic acid would do the trick? Maybe a friendly pharmacist could order a bottle for you - I remember 2 litre bottles full of it in my chemistry lab days, but I don't know how easy it is to buy such things these days.

 

[jess marley]

The house is fine, it's me that has accelerated erosion.
I'll try the chemist for acetic though.

 

[Gervase]

In the days when I did a lot of photographic printing, I used to get bottles of fairly concentrated acetic acid from Boots for use as a stop-bath. Probably on the banned list, these days, though - one request for it and MI5 will be going through your bins... :shock:

 

[biffvernon]

Vinegar and lemon juice just aren't going to be strong enough to shift thick limescale in a short time. Hydrochloric acid is what works - most easily available as the brick cleaning stuff sold at builders' merchants.

To check that your particular slates don't contain much calcite that would be damage test on a clean piece of slate. There should be no fizzing at all. Then you can be pretty confident to use it on the limescaled slates and watch them fizz. A wire brush can be used to remove the loosened scale long before it is all dissolved.

 

[andrew gayton]

I can't believe that caustic soda does not shift the paint...... it has always worked for me......

 

[jess marley]

Andrew, the caustic would absolutely take the paint off, you're quite right. My problem is the lime-scale underneath.
Thanks for that Biff, it was the composition of the slate I suppose I was worried about.

 

[andrew gayton]

eureka moment - i recently purchased a limescale removing product from the supermarket (the product that claims to be the best) and that removed the calcareous deposits from my bath taps...... in a blue plastic bottle....

 

[Lime]

eureka moment - i recently purchased a limescale removing product from the supermarket (the product that claims to be the best) and that removed the calcareous deposits from my bath taps...... in a blue plastic bottle....

I'm probably stating the obvious but keep acids away from enamel baths.
They fizz too!