I picked up some Virginia Lime Works Building Lime 150
and some fine marble dust in an attempt to skim coat the existing lime plaster wall with matching materials without reducing the water permeability by much. As predicted, I'm having trouble getting the new lime plaster to both stick and be smooth.
On my first try, I put a thin coat onto a piece of luan plywood I had sitting around. It worked pretty well. First, I sprayed the board with water. I mixed 1:1 lime to marble dust by powder volume into water until it was the consistency of mayonnaise. I put it on very thin. After 3 hours, I sprayed it down and troweled it almost smooth, though it did start to come off, so I stopped troweling. I figured that with a second coat, I would be able to get it perfectly smooth and polished.
Emboldened by this success, I soaked my wall the day before and again right before applying the skim coat. I think my mix was a little too thick. I added a bit of liquid soap because someone told me it made the troweling easier. The plaster became crumbly before I finished troweling it, so I just put the skim coat up quickly and left the trowel marks alone, hoping I could come back and fix it after it started to dry and set. Spraying and troweling the plaster before it's ready for touch-up just makes it wet and crumbly, while spraying and troweling after a day doesn't do much, especially for the bigger imperfections from mixing the plaster too thick. It seems very sensitive to timing and moisture level, and with me not wanting to work on it at 3am, I must have missed that window on the first try.
The plaster dried very hard with no cracking in about two days, but it was uneven, so I scraped it off. It basically fell off the wall where it was more than 1mm thick, while the thinner spots had to be scraped and chiseled off. Both the Virginia Lime Works plastering instructions
and the Master of Plaster product data sheets
say to apply in multiple "lifts" or coats that are as thin as possible.
For my second try, I wet down the wall again and applied a very thin coat of plaster that was closer to the first, wetter consistency. It dried out and crumbled even faster than my first attempt, despite mixing the plaster with more water. It had been a few days since the first attempt, and I think the wall needed more saturation. Also, trying to plaster that thin of a layer was difficult, leaving ridges perpendicular to the troweling direction. I went ahead and applied a second coat about an hour after the first, and the result was a much flatter, smoother coat, though it was still very finicky. I will test the adhesion when it hardens in a few days.
I also skimmed an area on the opposite hallway wall, which the previous owner repaired with drywall. The Master of Plaster product says it will stick to paint, and as I understand it, MoP is basically what I'm making, just with hydraulic lime instead of aged lime putty, and with no additives, while the MoP product has mica powder plus some other mineral called palygorskite and some other secret stuff. The skim coat over paint just turned to dust where it was very thin, which could be due to over-troweling, I don't know. I'll see about the thicker spots soon.
I'm encouraged by a few things. The areas with very thin plaster did stick. If I could get the first coat up fast, thin, and decently smooth, then the second and third coats would even out the remaining imperfections. Each coat does handle a little bit of troweling before it's completely set and a little more troweling later on, as long as you don't do too much at once. The wall could probably stand some more soaking with water, with a final spray-down right before plaster application.
Worse come to worst, I could just PVA the wall and then lime plaster, since it would still have better permeability than the painted wallpaper we just removed. It would be nice if there were a bonding agent that was water impermeable when applied and water permeable thereafter.
I'll keep trying things out. Any helpful hints on wall prep, plaster mixing, troweling, smoothing, polishing, and timing in general? Does a wood float work well on hydraulic lime? Is lime putty any better? Would it help to soak the wall with lime water rather than regular? Any additives I can use to improve the spread-ability, those of you with Venetian plaster experience? If I can manage to figure out a consistent method for an economical skim coat of new lime straight onto old lime, I imagine that would be helpful to someone.