What is best way to thoroughly clean airless sprayer after using shellac primer
Ammonia:

"…the objective is to remove the shellac from the brush, roller, etc, not to thin the shellac that is still in the brush.

Now, the explanation; just as mineral spirits/paint thinner won’t completely remove varnish from a brush, neither will alcohol alone completely remove shellac from a brush.  Alcohol will dilute the cut of shellac but there will still be some shellac still in the brush.  Ammonia breaks down the shellac so that washing the brush with mild detergent and warm water, after a soak in ammonia, will completely remove all traces of the shellac.  When we clean a varnish brush with paint thinner we are only thinning the varnish; some of the varnish remains to cure in the brush.  But, when we wash the brush in warm water and detergent (after cleaning in MS/paint thinner) the soap “grabs” the remaining molecules of varnish so that all of the varnish is rinsed from the brush…"