Porch plastering

[NickB_99]

Hello all,

I'm struggling with the plaster in my porch and hoping for some advice to correct.
A new front door (uPVC double glazed) was fitted in Oct, as the old wooden door wasn't good. One of the walls had taken a bit of damage when the old door was taken out, so I saw a good opportunity to have a go on a small wall - and correct later if all did not go to plan.

The original wall was the same as the rest of the house internally(blockwork with a grey browning coat with a thin grey skim of plaster. This had been painted with what looked like a silk emulsion)

I applied a 3:1 mix of PVA and when tacky, skimmed over the wall. The new multi-finish has held fine. (this was my first wall and with a budget trowel - my pre-marshalltown days - - I have improved! )

porch plaster

• NickB_99
• 7 Apr 2009
• 1

First try in the house - nasty! Too much water

The problem I am having is the new plaster has not dried out completely. There are a handful of slightly darker patches and am now starting to get a bit of 'salt', as you may be able to make out in the close up

porch plaster - close

• NickB_99
• 7 Apr 2009
• 1

There are prob about 4 or 5 small patches - between the bottom and no higher than about halfway up.

The porch is not heated, and in the winter the double glazing picked up quite a bit of condensation. The wall shown here is an internal one and behind it is the front room.

I am wondering if the problem is likely to be condensation - and the uPVC
has now reduced ventilation also. There is no air brick or anything.
I've read some of the posts regarding condensation etc. There hasn't been anything noticeable on the double glazing in the last month or so.
So I don't think the test advised in the posts on condensation (to stick some clear polythene to the wall to see which side condenses etc) would work at the moment.

Is condensation likely to be a cause of patchy damp spots like this?

any suggestions to fix? Take the hit and add an air brick (the opposite wall to this is external)? Get a trickle vent put into the uPVC frame on the windows to the side of the door? Can you brush it down and are there any water proof paints that would cover it (assuming it dries in the summer months)? D&D foil backed PB?

Any advice appreciated as always.

 

[Richard C]

Is it a single skin wall? If so is it outside exposed to the elemets or inside a garege?

 

[NickB_99]

Is it a single skin wall? If so is it outside exposed to the elemets or inside a garege?

The wall in the picture is an internal one between the front room and porch. I think it is a single skin, but block, not brick.
The opposite wall is still the original paint/plaster finish and a nice lemon colour in the porch .
This 'lemon' wall is normal brick on the outside and a plaster finish inside. Looks to be about 26cm thick (inc internal plaster) (is this enough for a cavity and poss brick & block?). Having had a closer look at this wall, it also looks like some flaky paint is near the bottom RHS, just above the skirt. Don't know if this helps.

If so is it outside exposed to the elemets or inside a garege?

If you were looking at the porch in Plan view and the top was the uPVC door, then the only other external wall is the RHS (the lemon one).

There is an internal glazed door that separates the porch from the rest of the house.

 

[Richard C]

My immediate thought was damp but if it’s an internal wall, that’s unlikely. Vinyl emulsion needs prepping before plastering & this recent thread list a few ways of achieving this;
//www.diynot.com/forums/viewtopic.php?t=174551

A skim should appear dry within a couple of days at most but there will still be a high moisture content so it shouldn’t be painted for around another 10 days. Ventilate the porch whilst the plaster is drying out; leave a window or the door open; or leave the internal door open, this will allow some gentle heat & dissipate the moisture as it evaporates but open a window nearby.

It’s difficult to asses but the mottled white finish in the first photo is the sort of thing you get when the plaster has been wetted too much when troweling up, did you use a lot of water on it? Never apply water as your laying the plaster on, it should only be used during the final troweling up stage & then as little as possible; many use a bucket & brush but I use an old 1 litre kitchen spray bottle &, as an example, I can skim all 4 walls in an average size room & still have water left in the bottle at the end! Use too much & the surface of the plaster breaks down leaving a white powdery mush that just sits in the hollows on the surface. Not sure about the salty crystals; it may be trapped moisture if you watered the plaster as you were laying it on unless it’s some sort of reaction with the paint; did you skim in 2 coats? How thick is the finish skim you put on?

As for the other wall; correct & thorough preparation is very important & this will vary depending on the type of wall & any paint finish that may be on it. Any loose or flaking paint should be removed & the surface primed or the new skim won’t adhere properly. Plaster won’t survive permeating damp for very long & the best solution with such walls is to line with Duplex (foil back) boards & skim those.

 

[NickB_99]

A skim should appear dry within a couple of days at most but there will still be a high moisture content so it shouldn’t be painted for around another 10 days. Ventilate the porch whilst the plaster is drying out; leave a window or the door open; or leave the internal door open, this will allow some gentle heat & dissipate the moisture as it evaporates but open a window nearby.

The wall was initially skimmed in Sept/Oct. It dried initially, but seems to have not got past the point it's at now. The crystals etc on top are something I've noticed recently and definitely post Christmas. Will go for the dry-out as suggested. There are no windows in there unfortunately. Only front door and internal door to house. Leaving the latter open will be possible.

It’s difficult to asses but the mottled white finish in the first photo is the sort of thing you get when the plaster has been wetted too much when troweling up, did you use a lot of water on it? Never apply water as your laying the plaster on, it should only be used during the final troweling up stage & then as little as possible; many use a bucket & brush but I use an old 1 litre kitchen spray bottle &, as an example, .......did you skim in 2 coats? How thick is the finish skim you put on?

Only used water when troweling up, but prob used too much. Initial exhuberence! I generally use a bucket and brush, and have tried to cut down since! Skim was 2 coats of about 1 mm approx each.
The same paint has been used on other places in the house (it's such a nice shade!). These have been skimmed with no problems. If you go plaster-paint-newplaster, does the damp generally still permeate through from the back?

As for the other wall; correct & thorough preparation is very important & this will vary depending on the type of wall & any paint finish that may be on it. Any loose or flaking paint should be removed & the surface primed or the new skim won’t adhere properly. Plaster won’t survive permeating damp for very long & the best solution with such walls is to line with Duplex (foil back) boards & skim those.

Will prep the other wall better. Tho with just the 3:1 PVA alone, it doesn't seem to have had a problem staying up. Will go with a better prep as you suggest regardless. Best be safe!
Can foil boards be D&D-ed? Wouldn't want to introduce too much thickness, or I will make the frame of the glazing look too thin on the inside.

 

[Richard C]

The wall was initially skimmed in Sept/Oct. It dried initially, but seems to have not got past the point it's at now. The crystals etc on top are something I've noticed recently and definitely post Christmas. Will go for the dry-out as suggested. There are no windows in there unfortunately. Only front door and internal door to house. Leaving the latter open will be possible.

I can’t see any way a plaster skim would not have dried out by now unless you have a damp problem in the wall or moisture is getting it in some other way.

Can foil boards be D&D-ed? Wouldn't want to introduce too much thickness, or I will make the frame of the glazing look too thin on the inside.

I had a single skin brick wall between the garage & a new porch & was also restricted due to the large front window. You can’t D&D foil backed boards but what I did was to fix them directly to the single skin wall from the front using “through” fixings & then plaster skimmed as normal. The porch has been in use for 3 years now & there heave been no problems with damp.

 

[NickB_99]

I can’t see any way a plaster skim would not have dried out by now unless you have a damp problem in the wall or moisture is getting it in some other way.

Definitely agree - though can't work out where from

I had a single skin brick wall between the garage & a new porch & was also restricted due to the large front window. You can’t D&D foil backed boards but what I did was to fix them directly to the single skin wall from the front using “through” fixings & then plaster skimmed as normal. The porch has been in use for 3 years now & there heave been no problems with damp.

That sounds like it would be worth a try and would not add too much thickness. The walls are only small - each approx 1.5m wide, so wouldn't cost much
How thick are foiled backed PBs? Are they typically 12mm only or do you know if it is possible to source thinner ones?

Thanks for your help thus far Richard.

 

[Richard C]

How thick are foiled backed PBs? Are they typically 12mm only or do you know if it is possible to source thinner ones?

I used 12.5mm but you can also get 15mm. 9.5mm (if it were available) would be marginal IMO & I wouldn't use anything less than 12.5mm.

 

[NickB_99]

Thanks for the help Richard. Thorough as always.