Not a good choice for rockets.
Shellac is an indoor product. Very yellow by nature and over time curls back on itself forming tiny balls or knots giving the object a rough almost stuccoed look. Mostly used on OLD furnature. Any Varnish would be a better choice, Polyurethane clear even better.
Both shellac and varnish are Pine tar products, I understand there are now a few synthetic versions but have no experence with those at all.
Hope this helps