The sun is burning and the heat is certainly on here in Denmark. This makes it a little harder to keep shohin-bonsai healthy. The warm weather is one thing, the sun adding heat to the pots is another. Trees transpire to cool down the temperature in their cells. Therefore sufficient watering is essential on these hot days. Watering during the hottest part of the day sprinkling the trees will on shorter terms cool down the trees a little, but it only lasts short. It wont harm the trees at all being wet with leaves exposed for the sun. Old stories about leaf burns because of water on the leafs acting like sun through a magnifying glass are not true. This is tested and proven wrong, so just go ahead cooling the trees with a little water sprinkling in the sun.
On the longer term, that means all day long, cooling the trees are also possible in other ways. Hanging up a shadow net to diffuse sun light is one method. Or, as I do, placing the trees in partly shadow provided by larger trees is another. Garden trees are placed on purpose to make some semi shade during the hottest part of the day, helping the trees from being over heated.
One more factor of importance is the pot and the clay quality and fabrication. Japanese pots often seems thinner in their construction of clay walls than many western pots. This is to avoid the roots being overheated by a thick pot which doesn’t cool down as fast as a thinner pot. Also the transpiration of the clay on non-glazed pots for conifers are an element of importance. Especially conifers prefers unglazed pots because they like their roots in a cooler environment. Deciduous copes with glazed pots a little better, and glazed pots are not transpiring through the glazed clay walls of the pot, therefore unable to cool the same way as unglazed bonsai pots.