External Lime Rendering Coats

[aockenden]

Hey all, I’m making my first attempt at exterior lime rendering (a cement-block gable wall). Here’s where I got to yesterday, however I’m not sure the best course to take from here: I definitely want to get a smoother final finish than this.

This first coat has stiffened up considerably overnight to the point that it’s no longer easy to smooth with a trowel, it just goes gritty and starts getting crumbly / falling off the wall. It would probably get moveable again if I soaked the wall thoroughly but idk if that’s the best approach.

So the essence of the question is: should I be trying to smooth out this first coat to the point that it’s ready for a limewash, or should I leave this coat as-is and instead apply a second coat to really smooth it out?

 

[Cubist]

In my opinion that ships sailed. Soaking it will weaken its grip and may cause the initial render coat to fail and trying to polish it now would almost certainly fail too. Maybe scratch the surface to provide a key for a final top coat?

 

[aockenden]

In my opinion that ships sailed. Soaking it will weaken its grip and may cause the initial render coat to fail and trying to polish it now would almost certainly fail too. Maybe scratch the surface to provide a key for a final top coat?

Yea ok that’s what I was leaning towards anyways. Couple questions about the next coat then:

Should I add a fiber? All I’ve got on hand is sheepswool and hemp. What ratio of sand to lime? And I assume it’s best to get it on the wall ASAP before the first coat is fully dried so the two coats can gel together?

 

[Stevers]

Will it still scratch to get a better key for the top coat?

 

[Cubist]

Yea ok that’s what I was leaning towards anyways. Couple questions about the next coat then:

Should I add a fiber? All I’ve got on hand is sheepswool and hemp. What ratio of sand to lime? And I assume it’s best to get it on the wall ASAP before the first coat is fully dried so the two coats can gel together?

Frankly, I've no idea. No direct experience of using haired lime in any context.

However, I would ordinarily point to Stevers, Kearn, Lacan or maybe Zebra as people who may have direct knowledge.

That aside; I'm surprised the recent downpours we've had don't seem to have had a negative impact. That and its getting a little late for external render work isn't it?

BTW what part of the country are you in anyway?

 

[aockenden]

Frankly, I've no idea. No direct experience of using haired lime in any context.

However, I would ordinarily point to Stevers, Kearn, Lacan or maybe Zebra as people who may have direct knowledge.

That aside; I'm surprised the recent downpours we've had don't seem to have had a negative impact. That and its getting a little late for external render work isn't it?

BTW what part of the country are you in anyway?

Fair enough, hopefully they’ll chip in their 2 cents.

We’re in mid-wales. Ive got 3 days of forecasted dry weather here as a window to get-er done. As for it being late in the year, I’m really bad at following these sorts of “rules” and when I’ve got time off work between screaming fits from my 2-year old son I have no choice but to just take my shot and hope it works ok. We thankfully don’t have any frost in the near-term forecast.

 

[aockenden]

Will it still scratch to get a better key for the top coat?

Yeah probably, think it needs it?

 

[Cubist]

I think you'll need some extra 'grab' to butter that cake so I'll anticipate Stevers and say yes.

Did you know that a study was commissioned by the forerunners of B&Q that established a direct correlation between the age and number of screaming kids in a household and the DIY spend of those homes? I may have just made that up but.... perhaps worth considering alongside the other planning requirements.

 

[aockenden]

I think you'll need some extra 'grab' to butter that cake so I'll anticipate Stevers and say yes.

Did you know that a study was commissioned by the forerunners of B&Q that established a direct correlation between the age and number of screaming kids in a household and the DIY spend of those homes? I may have just made that up but.... perhaps worth considering alongside the other planning requirements.

Ok just did it, thanks. Haha l’ll keep that in mind…

 

[paulc]

Should I add a fiber? All I’ve got on hand is sheepswool and hemp. What ratio of sand to lime?

I've used polyester screed fibres mixed in to lime plaster. Several suppliers of ready-mix plaster/render use synthetic fibres as they don't break down in the wet mix like animal fibres do. Also used short fibres (2mm) mixed in to a fine finishing plaster, but found it difficult to get an even dispertion, so one ends up with little balls of polyester. The longer 20mm fibres mix in OK, and being a base (or even a second of three) coats, it doesn't matter too much if strands stick out or clump slightly.

 

[aockenden]

I've used polyester screed fibres mixed in to lime plaster. Several suppliers of ready-mix plaster/render use synthetic fibres as they don't break down in the wet mix like animal fibres do. Also used short fibres (2mm) mixed in to a fine finishing plaster, but found it difficult to get an even dispertion, so one ends up with little balls of polyester. The longer 20mm fibres mix in OK, and being a base (or even a second of three) coats, it doesn't matter too much if strands stick out or clump slightly.

Ok well I’ve teased in some coarse sheepswool to this second coat. It’s going on pretty nicely at the minute. I figure it can’t hurt much.

 

[aockenden]

K slapped the second coat with sheepswool fiber onto the prior coat and smoothed it with a sponge float and flexible plastic trowel. I think it’s reasonably good

 

[Kearn]

Little point wasting fibres now - useful in the scratch coat to resist movement in the substrate. You’ll doubtful have much movement anyway onto block.

Scratch it - use a bit of timber with nails through, about an inch apart. That’ll give a key for another go. It doesn’t want to be too wet when you go on with the next coat otherwise you won’t get eight suction to pull the top coat in.

I’m assuming you’re using NHL? What number, what size aggregate and what ratio?

I’d say your consistency looks ok, possibly a bit dry to trowel, but should’ve been ok to get a smooth troweled finish.
However, in the nicest possible way, the next coat won’t really make much difference if you don’t change your style!

What tools are you using?

An alternative to a large flat metal trowel, which I find hard to finish with, is a sponge float. More forgiving, but will bring a bit more aggregate to the surface. Personally I like the appearance. Just don’t overwork it or you’ll pull too much and the line will dry too fast.

 

[aockenden]

Got another wall going today. Haired / wooled the first coat this time. Will assess for a second possible coat tomorrow.

 

[Cubist]

Looking good. Also looks like that shed/out-house is built to last - refuge from a planned houseful of screaming kids perhaps?

 

[aockenden]

Looking good. Also looks like that shed/out-house is built to last - refuge from a planned houseful of screaming kids perhaps?

Lol yeah bingo. Thanks, I’ve rebuilt it from the rotted wall-plates right the way up. New wall plates, new trusses, extended gable blockwork up to meet the trusse, built the overhang ladders, did the felt, battens, slates, ridge tiles, barges, fascia’s, soffits, loft insulation, knocked out hole for a first window and bricked up a second doorway for a second window, painted and installed windows (built by our joiner custom), did the tng ceiling inside, started building the hearth for a woodburner, now rendering externally. Still need to install custom door on order and do internal insulating plastering and reclaimed slate flooring. I’m really tired.

 

[Cubist]

We're not talking about that out-house anymore right?

 

[aockenden]

We're not talking about that out-house anymore right?

Yes that’s everything I’ve done on that outbuilding over the past 3 months.

 

[aockenden]

Little point wasting fibres now - useful in the scratch coat to resist movement in the substrate. You’ll doubtful have much movement anyway onto block.

Scratch it - use a bit of timber with nails through, about an inch apart. That’ll give a key for another go. It doesn’t want to be too wet when you go on with the next coat otherwise you won’t get eight suction to pull the top coat in.

I’m assuming you’re using NHL? What number, what size aggregate and what ratio?

I’d say your consistency looks ok, possibly a bit dry to trowel, but should’ve been ok to get a smooth troweled finish.
However, in the nicest possible way, the next coat won’t really make much difference if you don’t change your style!

What tools are you using?

An alternative to a large flat metal trowel, which I find hard to finish with, is a sponge float. More forgiving, but will bring a bit more aggregate to the surface. Personally I like the appearance. Just don’t overwork it or you’ll pull too much and the line will dry too fast.

My mix is 1 part putty, 1 part NHL 2 powder, 4 parts sand. Plus sone sheepswool now. I’m adding more water than that first coat which is making things easier as well.

I used a medium grain sponge float on the second coat and I can’t say I’m a fan tbh. It exposed the aggregate more than I thought it would. On that first coat I posted pics of I used a steel trowel but I’ve now switched to a flexible plastic trowel which is working much better for me.

 

[Kearn]

My mix is 1 part putty, 1 part NHL 2 powder, 4 parts sand. Plus sone sheepswool now. I’m adding more water than that first coat which is making things easier as well.

I used a medium grain sponge float on the second coat and I can’t say I’m a fan tbh. It exposed the aggregate more than I thought it would. On that first coat I posted pics of I used a steel trowel but I’ve now switched to a flexible plastic trowel which is working much better for me.

Think it’s a bit of what works best for folks. I’ve seen perfectly flat finishes with a metal trowel, but that’s the pros… it’s one of those tools that in the right hands is perfect, but shows up those of us without decades of experience! I can’t make it work…yet!

A bit like buying an expensive espresso machine and grinder… initially your coffee gets way worse than with the cheaper kit!!