  A general formalism for covariant $W_3$ string scattering is given. It is
found necessary to use screening charges that are constructed from the $W_3$
fields including ghosts. The scattering amplitudes so constructed contain
within them Ising model correlation functions and agree with those found
previously by the authors. Using the screening charge and a picture changing
operator, an infinite number of states in the cohomology of Q are generated
from only three states. We conjecture that, apart from discrete states, these
are all the states in the cohomology of Q.
