The cedar shingles don't have to be bare. The purpose of washing is to remove dirt, dust, pollution and maybe some loose paint. As you found out, trying to strip the paint with the pressure washer is dangerous.
The shingles will dry out after a couple of days. The only definite way to tell if they are dry and to what point is to use a moisture meter. Just wait a couple of days. If the singles feel wet or are rewetted wait a little longer.
As you are waiting for the singles to dry, you can start looking for loose paint and caulking. Remove the loose material before priming.