Last summer we were working on a job skim-coating a clay plaster over drywall, but the drywall prep was uneven and fairly rough. We were having trouble with joints showing through.
The fix for this is relatively simple – apply the plaster in two thin coats. The first coat helps level the wall, and deals with uneven suction (one part of the wall is absorbing water faster than other parts, which can cause texture changes in the finish). The second coat is applied while the first coat is damp, but not wet or sticky. Typically the second coat can be applied within an hour of the first coat, but this will vary depending on drying conditions. Both coats are applied quite thin. Two-coat plastering is also a good trick for novice plasterers who are having trouble with even application depth. And two-coating is a good way to deal with very absorbent substrates, such as thick plaster base coats. Skim-coating over a very absorbent base is very challenging, even with heavy misting. A first coat applied very thin and fast helps break the suction and allows a little more time to apply and finish the second coat.
Another trick if you’re having trouble with thin finish coats is adding invisible fibers. They can help with workability of the plaster, and deal with shrinkage cracks etc. I’ve always loved fine hemp for this, but I’ve been having trouble sourcing good quality hemp fiber. My new favourite fiber is cat tail fluff, which can easily be gathered for free. The stalks can be gathered at almost any time of year, but late fall or winter are usually the best seasons, when the fluff is mature and loose but not exploding off the stalks. However I’ve found usable stalks from the year before even in mid-summer when the new stalks are forming.