I use acetone rather than alcohol for spirit sensitizers but both will
dissolve (or melt) the surface of the pigmented gelatin if either the
sensitizer or the working room is too warm (over 65F). This may not be
readily apparent when sensitizing brown and black tissue in subdued light,
but it is very evident with three color tissue if the same brush is used to
sensitize all three colors. If for example you brush over the cyan tissue
first, then the yellow, you will see very definite traces of cyan color
that are carried over.
It is much easier to avoid this melting of the gelatin with tray
sensitizing (just make sure that the sensitizer is very cool, even cold).
Also, I find that tray sensitized tissues are of a more consistent printing
speed.
Sandy King