Hi You can pressure wash them but be very careful and don't stand too close, also i would only do this if they were very dirty, we only use horse shampoo on bottoms, the rest of the body we use just cold water as you will strip all the natural oils from the fleece this can make the fleece even harder to card up. Do your washing at least a week before the show to allow the fleece time to recover. It also depends on what breed you have, our show hampshires are carded well then trimmed up to make them look bigger and tidy for the show ring....the more you card the bigger they can look but some judges can see through this so if your sheep looks good with just one carding ...i would leave it at that. good luck
To use the carder push the hooks into the fleece and pull in a upwards motion so the wool fluffs up, done quickly is better than pulling slowly which can annoy the sheep. Our sheep stand quite still for this so maybe they like the sensation!