Plastering over a thin sand/cement parge coat.

[epsilonGreedy]

Before I begin I should clarify that the type of parge coat I am talking about is a very thin sand/cement layer smeared over an internal block wall for air tightness. I mention this because I know that our resident plastering pro @nod sometimes refers to a parge coat when discussing a thicker scratch coat layer.

 

Anyhow onto my question. If I smear a thin sand/cement coat over my block walls and later switch plan from dot & dab plasterboard to a classic wet plaster finish, will the proper plaster scratch coat adhere ok to the thin smeared coat that has sealed the block surface? I ask because I imagine the original block surface offers better grip when a block wall is wet plastered.

 

Motivation: As I am prepping a small section of my unfinished house to create a site office I have noticed how dusty the environment is and I am hoping that applying a thin smeared parge goat over the blocks will lock in masonry dust escaping from the blocks. The proper plastering work to finish the room is a year+ away.  

[PeterW]

Yep will be fine unless you polish it hard. Sweeping brush and a sloppy 3:1 mix will be fine. You won’t get it silky smooth and your bonding coat will cover the sins later 

[markc]

Unless you manage to get the sand cement coat to a polished finish then plaster will stick ok.

[Russell griffiths]

As above. 

But this will not stop your walls being dusty, the parge  coat will dry and as you rub against it will dust up and bits fall off, for the cost of 3-4 sheets of plasterboard why not just slap up a couple of sheets and chuck a coat of emulsion on them. 

 

Better finish, will reflect the light rather than absorb it. 

 

[tonyshouse]

Wanting a good airtight house one would avoid dot and dab at all costs 

[Oz07]

1 hour ago, tonyshouse said:

Wanting a good airtight house one would avoid dot and dab at all costs 

Not true if parged and detailed correctly

[tonyshouse]

? and therein lies the problem. Too many ways to fail including under the floor, over the ceiling, round services, in theory it can be done but I haven’t seen one yet 

[joe90]

With mine I parged only in the space between downstairs ceiling and first floor flooring, the scratch coat on the walls both downstairs and up does the parging  itself so no need to parge those areas IMO.

[Oz07]

13 hours ago, tonyshouse said:

? and therein lies the problem. Too many ways to fail including under the floor, over the ceiling, round services, in theory it can be done but I haven’t seen one yet 

Mines been done like that. Score just over 1 on the air test. That's good enough for me

[tonyshouse]

We can do it yes but they can’t- the volume builders that is 

[epsilonGreedy]

On 23/04/2021 at 16:45, markc said:

Unless you manage to get the sand cement coat to a polished finish then plaster will stick ok.

 

 

Ok yesterday did not go so well. 

 

I mixed up half a bucket full of quote @PeterW"sloppy 3:1 mix will be fine" and applied it to a test 2 m2 area of wall. The floor had been swept clean which is a good thing because half the sloppy mix ended up on the floor and I was able to reuse it. This was a failure in technique.

 

Today I have concluded my finishing options are:

1. A very wet slurry finish that half fills the pores on the surface of a block, this will be little more than 1mm thick.
2. Slightly stiffer mix applied more like plaster and thick enough to fill the bucket handle mortar pointing. I think this will be 2 to 3mm thick.

Half way through the trial patch I found a rectangular plastering trowel in the shed with a course square-tooth serrated edge which was much better than a standard brick laying trowel. This leaves a thicker finish but the parge coat dried very quickly so my attempt at a brushed finish just knocked off the square pattern left by the plastering trowel leaving a smooth finish. Not a glassy smooth finish however.

 

If I am going to wet plaster a year later, this venture feels like wasted effort. Perhaps @Russell griffithsis right and the parge coat will be almost as dusty as the current block finish. I might just line out this 8' x 8' space with panels of osb or plasterboard.

 

When I started this job I imagined a finish similar to the smoother sections of the beam & block floor finish that had a very wet & strong mix to lock the blocks in position. The areas of the block and beam floor that were not later contaminated with 5 to 1 mortar dropped by the brickies, can be scrubbed clean to a dust free finish and I had imagined a similar parge coat finish on the block walls.

 

If I do continue with the parging, would finish (1) be ok for later plastering if the slurry mix half fills the rough finish in the surface of the blocks?

 

[bassanclan]

I applied parge slurry with a long handled broom

[joe90]

Why do you want to parge the walls if your going to wet render/plaster them anyway?

[epsilonGreedy]

42 minutes ago, joe90 said:

Why do you want to parge the walls if your going to wet render/plaster them anyway?

 

 

As per my initial post.

 

On 23/04/2021 at 16:37, epsilonGreedy said:

Motivation: As I am prepping a small section of my unfinished house to create a site office I have noticed how dusty the environment is and I am hoping that applying a thin smeared parge goat over the blocks will lock in masonry dust escaping from the blocks. The proper plastering work to finish the room is a year+ away.  

 

 

 

[epsilonGreedy]

1 hour ago, bassanclan said:

I applied parge slurry with a long handled broom

 

 

Did you wet the walls before brushing on the slurry? I found that within seconds further brush strokes causes the quickly drying slurry to crumble off the wall as small granules.

[joe90]

45 minutes ago, epsilonGreedy said:

 

As per my initial post.

 

 

 

Oops, missed that bit ?

[Russell griffiths]

2 hours ago, epsilonGreedy said:

 

Did you wet the walls before brushing on the slurry? I found that within seconds further brush strokes causes the quickly drying slurry to crumble off the wall as small granules.

Yes, don’t fanny about, get a big bucket and a soft broom, soak the walls from the top down, proper soaking needed. 

[Oz07]

I used a stiff brush and destroyed 2 on mine. It kills the bristles 

[Oz07]

Also 3 and 1. @PeterW must be getting his cement like he gets everything else...cheap as fu*k

I was more like 5 maybe 6 definatley not 3

[PeterW]

1 hour ago, Oz07 said:

Also 3 and 1. @PeterW must be getting his cement like he gets everything else...cheap as fu*k

£4.32 a bag .. ? Parge coat shouldn’t use much anyway tbh

[epsilonGreedy]

10 minutes ago, PeterW said:

£4.32 a bag .. ? Parge coat shouldn’t use much anyway tbh

 

 

I had 10 bags of cement sitting on a wooden pallet over the garden lawn during the winter. Plastic bags and under a tarpaulin. Still powder dry yesterday.