I've read the thread on removing paint from rough sawn wood but it doesn't exactly address my situation. My daughter painted some designs on the ceiling of her room with some kind of tempera-based (I think) fluorescent paint. the ceiling is rough sawn red cedar and unfinished. I don't plan (or would prefer not to) paint the ceiling as it is a log house and the bare wood look is something I want to keep. I am concerned to find a method of removing these painted spots with minimal impact to the color or texture of the boards. So, for example, if there were some method of wetting the paint and soaking it out, that would be great. Alternatively, I am considering wire brush, but I think that will make a noticeable difference in color and texture of the wood, which has naturally darkened over the years.
I suspect Graphus may have some insight on this.
Tempura paints are waterbased and should dissolve with soap and water. I would start by wire brushing it to remove as much dried pigment as possible. I would then attempt to remove it by gently wetting it with soapy water repeated several times. You may be able to reduce the risk of pigment soaking in further repeated rinsing and vacuuming it up as you go. The fact that the surface is upside down on the ceiling may help draining the water away from the board as you go. I suspect however, that some staining will remain and, in the end, the texture of the wood surface will differ from untreated areas. Even if you remove the pigment, the water stains may leave a visual stain on the board. If so, you may have to wet the entire area to get an even appearance when it dries.
If you are very concerned, you may want to create a test panel duplicating the problem, let it dry for a few days and then test cleaning approaches on it.