DL
After reading the postings on this subject, I offer the following to further cloud the picture. If you find that this is too much information ......... I apologize ahead of time. I have found that when a health department requires wall surfaces to be easily cleaned they mean that it should be smooth enough to be wiped down like a hard tile surface. I don't think the lime washed, sand float finish you are contemplating will give you that but a smooth, hard-troweled finish will.
Wood lath over studs spaced at 24" o.c. should probably be no less than 3/8" thick with a 1/4" - 3/8" gap, depending upon the size of your aggregate and the type of fiber that is in the base/scratch coat. For a lime scratch coat that would most likely be hair, hemp or even chopped straw.
I would not recommend that you use a clay based scratch coat in your plaster system. To get the best bond possible between the lime finish and the clay base requires several layers of plaster of different combinations with the clay and lime. Probably more work than the overall job warrants. I'd suggest you consider a 3-coat lime plaster with an overall thickness of 5/8" - 3/4".
Plaster work is a little different than mortar work in that the hydrated lime actually needs to be mixed with water (lime into water) and soaked for a length of time to allow it to full absorb all of the water it can. Allow it to soak in a covered container from 1 to 3 weeks. During this time the lime particles will settle to the bottom and form a putty with a consistency that approaches cream cheese. As long as you keep the putty away from air and under water it will last indefinitely. Lime putty, like wine, gets better with age. A 50 lb. bag of hydrated lime mixed with about 10 gallons of water will yield about 8 gallons of lime putty. Those 8 gallons of putty can be used in the following mixes.
Scratch Coat - 50 sf of coverage @ 3/8" - 1/2" thickness mixed at a ratio of 1 part lime putty to 2 - 2 1/2 parts aggregate and fiber added.
Brown Coat - 62 sf of coverage @ 3/8" thickness mixed at a ratio of 1:1.75 - 2
Finish Coat - 180 sf of coverage @ 1/8" thickness mixed at a ratio of 1:1 - 1.5
Aggregate gradation is based upon the thickness of the plaster coat that is being prepared. If your project will not allow the time needed for the lime to properly carbonize and cure out then gauging it with white cement is always an option.
The information here is based upon my work with both clay and lime plasters that I am called upon to provide from time to time. I hope it is helpful.
