Painting the walls
Table of Contents
This is going to be epic. After all the stripping, patching and looking at these dark walls for two years, with their highly contrasting areas of new plaster and dark milk paint, the time has come to convert this masonary box into a room. I have to see what it looks like in white.
The plan is to lime wash the walls to retain their permeability, so a permeable prep coat needs to be applied to provide an evenly absorbent substrate.
After testing a small area with the water based Bauwerk Prep Coat, and finding no issues with the milk paint and the lime plaster patching, I went ahead and sprayed the walls. It sprayed well using the Wagner Pro 117 sprayer and the 315 tip with the pressure slider at about half way. It formed quite a thick, even and opaque layer (approximately 8 sq.m per litre coverage). I could see the patched areas underneath only because they absorbed faster and were less "wet". So far so good. I'll give it at least a day to dry with the inside temperature around 15°C.
This one prep coat would have been enough but ...
Patchy, contrasty walls
2. First Prep Coat sprayed
We have a problem
When the gyproc lining was demolished I discovered wallpaper had covered the walls and had been painted over. This wallpaper has all been removed, and even though the walls had been washed with a warm sugar soap solution at least once (twice in some areas) prior to prepping, to the point where the pattern was no longer visible, there is clearly chemistry still present that has been activated and is now visible through the Prep Coat, which is not designed to be a blocker as it is permeable. Image 3 is actually a section of the wall where the wallpaper had been removed before the room was lined back in 1983, so whatever it is it's going to be persistent.
There is also staining coming through generally, I believe from the milk paint itself.
It looks like the only solution here is to apply a moisture blocker. This is a disappointment but in reality it's not a disaster. The exterior walls will always be permeable to the outside, so moisture will not become trapped in those walls. Elsewhere, walls that are both interior and have been impermeable on both sides for at least four decades do not show any deterioration.
3. Close-up of wallpaper pattern bleed-through
4. Contrast enhanced version of image 3
A few different treatments have been tested, including a second prep coat, diluted PVA before a second prep coat, and an untinted lime wash coat. These are all water based and none of these fix the problem. Indeed, one test (the lime wash) made the problem doubly worse and is the area shown in image 3. That is significant because three layers of lime wash will be going on the walls. My conclusion is, if water can get to the milk paint layer the staining will keep coming back through.
I know that the shellac based primer used on the ceiling worked there but I'll test a water based moisture barrier paint for comparison. Simply because it was in stock locally I'm testing Cromelin's Aquablock, applied over the existing Prep Coat. I would have tried Zinsser's Smart Prime as well but the local Bunnings was out of stock of the 1L cans and only had the 1 US gallon (3.78L) can, so I bought the Cromelin 4L can because it was 50% cheaper than the Smart Prime. I will use it somewhere else if it doesn't work out.
One coat of Cromelin Aquablock by brush (image 5 lower half) did not completely block the pattern but that may be normal. I noted that the Zinsser Smart Prime product states that this is normal for it but assures that the staining will be locked to that layer. That's what I would expect also. Cromelin says a second coat of it's product may be required.
I applied the prep coat over the single coat test area and after several hours drying the pattern can partially still be seen through the prep coat, although it's less prominent. But I don't think I'll chance any water based blocker.
I've repeated the same test (image 5 upper half) with the shellac blocker and no bleed-through is evident to the naked eye, so it's looking like the Zinsser BIN blocker will be it.
5. Contrast enhanced test area
Revised paint scheme
I'm going to lose permiability however I see no other practical guarranteed solution so, applied over the top of the previous Bauwerk Prep Coat layer will be the following:
Zinsser BIN Shellac Primer, tinted light grey to make any missed areas stand out, rolled.
Bauwerk Prep Coat, sprayed.
Bauwerk Lime Wash, brushed, at least two coats, with a third coat if the colour and effect warrant it.
Progress can be seen in images 6, 7 and 8. There's no evidence of bleed-through.
6. Shellac coat
7. Prep coat
8. One coat of Bauwerk Prep Coat sprayed over the shellac coat