[simon w]

We pressure cleaned this drive twelve months ago no chemicals and have been asked to do it again asap. I am thinking of treating it with hypo after but I'm wondering if these are the type of slabs that will suffer rust stains if treated with hypo? If hypo can be used would it be a good idea to leave it on the slabs after it's been applied without rinsing away with water?

[Smudger]

You will be fine with Hypo - most problems arise when using acids on natural stone - hypo is alkaline

I would use a strong solution on those blackspot areas 5050 mix - leave for 20 mins - re apply if required then give a light rinse over when your happy with the result

Darran

[simon w]

You will be fine with Hypo - most problems arise when using acids on natural stone - hypo is alkaline

I would use a strong solution on those blackspot areas 5050 mix - leave for 20 mins - re apply if required then give a light rinse over when your happy with the result

Darran

Thanks Darran. It'll look a lot better for using it. I've done a few recently where I've applied hypo to the whole patio after it's had a full pressure clean so there's no dirt left only black spot etc and not rinsed the hypo away afterwards at all. I have returned the following day and they have looked good for it. Obviously if there are lots of plants and grass surrounding the hard surfaces it's a good idea to rinse away after it's worked but are there situations where you just apply hypo and leave it on? Got a small section of roof to do tomorrow and after a low pressure clean I'm planning on applying hypo and leaving it on.

[Smudger]

Yes generally hypo is the last thing down, I only rinse if used neat and on an area where the customer is going to be walking straight away

I never rinse down roof work

Advice above was if you had concerns about leaving hypo down, but with black spot I don't leave until it's gone on the day, even if I don't rinse afterwards

Darran

[Kev Martin]

LTP do a black spot remover it is very very good

https://www.ltp-online.co.uk/blog/ltp-introduces-new-black-spot-algae-remover

[simon w]

You will be fine with Hypo - most problems arise when using acids on natural stone - hypo is alkaline

I would use a strong solution on those blackspot areas 5050 mix - leave for 20 mins - re apply if required then give a light rinse over when your happy with the result

Darran

Darran

I believe that hypo can cause corrosion and rusting on untreated metals and some stone slabs contain metals hence the risk of rust stains on stone slabs that contain metals? or have I got this wrong?

[simon w]

LTP do a black spot remover it is very very good

https://www.ltp-online.co.uk/blog/ltp-introduces-new-black-spot-algae-remover

I find Hypo works well, is easy to obtain and very cheap to buy Kev. What advantages would the product in your link have over regular 15% hypo?

[Smudger]

Yes it will attack untreated metal, but this happens over a period of time and if left in contact for long periods (like letting tools sit uncleaned in a strong mix - springs in my chem injectors tend to rot off again that's a combination of the guys not cleaning and full strength hypo passing through it usually about 2 years to break down a spring

Natural stone although contains iron has (ime) never been effected by hypo - that's not to say if you have a concern the best practice is to test first - acid on the other hand really attacks the metal immediately

Hope this helps

Darran

[Splash & dash]

You will be fine with Hypo - most problems arise when using acids on natural stone - hypo is alkaline

I would use a strong solution on those blackspot areas 5050 mix - leave for 20 mins - re apply if required then give a light rinse over when your happy with the result

Darran

Thanks Darran. It'll look a lot better for using it. I've done a few recently where I've applied hypo to the whole patio after it's had a full pressure clean so there's no dirt left only black spot etc and not rinsed the hypo away afterwards at all. I have returned the following day and they have looked good for it. Obviously if there are lots of plants and grass surrounding the hard surfaces it's a good idea to rinse away after it's worked but are there situations where you just apply hypo and leave it on? Got a small section of roof to do tomorrow and after a low pressure clean I'm planning on applying hypo and leaving it on.

Hypo will cause rusting if oxides exist in natural stone , we do lot of slate hung buildings and if there is any oxides in the slate it will go rusty , it doesn't do this with granite though , di a test patch first if it could be a problem .

[Kev Martin]

You will be fine with Hypo - most problems arise when using acids on natural stone - hypo is alkaline

I would use a strong solution on those blackspot areas 5050 mix - leave for 20 mins - re apply if required then give a light rinse over when your happy with the result

Darran

Thanks Darran. It'll look a lot better for using it. I've done a few recently where I've applied hypo to the whole patio after it's had a full pressure clean so there's no dirt left only black spot etc and not rinsed the hypo away afterwards at all. I have returned the following day and they have looked good for it. Obviously if there are lots of plants and grass surrounding the hard surfaces it's a good idea to rinse away after it's worked but are there situations where you just apply hypo and leave it on? Got a small section of roof to do tomorrow and after a low pressure clean I'm planning on applying hypo and leaving it on.

Hypo will cause rusting if oxides exist in natural stone , we do lot of slate hung buildings and if there is any oxides in the slate it will go rusty , it doesn't do this with granite though , di a test patch first if it could be a problem .

This because put in simple terms, slate is metamorphic and often contains Iron sulphides  and this is where the rust emanates from.  Granite is igneous and therefore unlikely to contain iron sulphides.  This is not to say that Granite doesn’t rust because it can, but it is usually caused by penetrating topically from a plant stand or similar.  If you use the Rust Remover I suggested in a previous post you won’t have the same problem as you do with Hypo.  The full problem is a little more technical  but this explanation covers most bases

[Roland S]

yes hypo will make some stone rust. some sandstones rust if you're not careful. it enhances the colours and streaks in some types - the red and brown streaks being basically rust to start with (ferrous oxide) - and leaves others with tiny little flecks of rust across the slabs that weren't there after cleaning but after applying hypo. and as someone else said, slate can be another one. so you just have to be careful sometimes and do a test patch if not sure.