If your farm tends to suffer occasional runs of overlay, there are probably several factors involved.
Overlay consists of a condition where the mycelium has reached the surface (just under the fine crumbs) and has started to grow horizontally. However instead of forming cords and pins, strands of mycelium fuses with other strands to form a continuous network or mat that no longer has any individual cords and tips capable of forming pins. The mat tends to get denser and denser.
If mycelium is sparse in the casing, there is less likelihood of it being able to form a mat. On the other hand, if there is ‘too much’ mycelium this is likely to make overlay more likely. Excess mycelium could be due to too high a case-run temperature and/or too much C.I. (CAC). In this respect straws brought up from below by deep ruffling would add to the amount of mycelium in the casing, and if it already had ‘sufficient’ mycelium derived from the C.I. it could make matting more likely. Relatively ‘dry’ casing favours more and faster mycelium growth.
If mycelium growth in the casing is rapid and/or the break is not adequate, (due to too high a CO2 level, or too high humidity, or to too high a temperature) these could be additional factors leading to ‘running on’/matting. In general, the faster the case run the earlier and more precisely timed the airing has to be in order to stop the run-on. For example, if you were speeding in a car you would expect to take longer to stop.
Finally and very important, if the casing is heavily watered, just before airing, this is likely to trap CO2 in the casing layer because CO2 is very soluble in water. If CO2 is trapped in this way it is likely to hinder the formation of pins. Late watering will also raise humidity in the microclimate that comprise the casing fissures, that the pins must form in. This is another hindrance to pinning. To make matters worse, if Sporgon is in the final watering, a high concentration in the pinning layer could just be the last straw as regard, formation of pins!
Growers who don’t know what overlay is, because they have never seen it on their farm, tend to take things easy. They employ a 19-21 day spawn run, and an 8-9 day case run, temperatures throughout are kept low.