I haven't seen solid-on-solid sweating so far. May have something to do with displacement - sweating will not happen if the game doesn't know how to displace the fluids without deleting any. This can often block seemingly obvious sweating sites. From what I can tell, sweating will always place the newly phase-changed material at the point where phase-changing material was (ie. 1000kg water will be displaced by 5kg steam), except in case of solids, where the sweating instead displaces t