It is easier to get the previously applied cob and the new cob to stick together if the moisture levels are similar. This means that you are riding the fine line between keeping the top of the wall as moist as possible so the next cob layer will adhere to it, and letting the lower part of the wall dry enough to support the weight of more cob.
Cover the tops of the walls with dampened straw, burlap sacks, and/or tarps and kiss it good night.
If you suspect heavy rain is coming, you can cover the walls with tarps or sacks. To speed up drying, you can place lots of dry straw on top of the wall under the tarps or sacks. If it's likely to be windy, weigh down the covers so they won't blow off. The rain won't hurt your wall seriously but it will, obviously, slow down the drying time.
It's a good idea to keep your fresh walls out of strong direct sun and harsh winds, which will dry the outside surface too quickly, and can cause cracks on the dried surface as the inner cob dries and shrinks. Protect the walls with shade or by covering them. If you're cobbing where the temperature drops below freezing at night, pile straw bales or loose straw on the sides and on top of the walls to prevent them from cracking up. You could use old sleeping bags or anything that insulates. Take the covers off during the day so the walls can dry.
If you'll be back to do some more cobbing in the next few days, stop working on the walls while you still have enough energy to make up some cob mixes to cure. It's a delight to have mixes ready at the start of a new cobbing day. Because these mixes will be curing and drying out for awhile, you can mix them extra wet which is easier and quicker.
I also like to tidy up so when I return to the site it feels welcoming, and I can find my clean tools.
It's as simple as that! Keep cobbing and sculpt your home!
It's OK. Fill the holes in the top of the wall again and again with water until the cob is rehydrated. Be careful to really incorporate the first new layer of cob. Use a stick and push the new cob hard into the old. Another trick is to sew the layers together with straw, pushing pieces of straw through the new mix into the holes in the old cob.
If you know that you'll be leaving the partially finished building for a long time, there are a few things you can do to make it easier to connect the next 'layer' of cob.
Plaster adheres beautifully to cob and will cover minor dips and smooth out the overall contours of the walls. But if you want to make a more major change and add new cob to the old, start by wetting the dry cob as much as possible. Drill, carve, or hack with a hammer claw holes in the hardened cob for the new cob to hold onto. You can hammer some old nails partway into the dry cob to help support the cob you're adding.
If you want to add a window or door, or change something in an already dry wall, get out the battering ram. You will soon have a new respect for the strength of cob and a healthy appreciation for taking the time to plan carefully. A 3/4 inch steel pipe can be sledge hammered through the wall many times where you want an opening. Wetting the wall again and again helps. Try an axe or hatchet to chop out what's left. It is much easier to fill in an unwanted door or window than to chop one out!