The nice thing about repairing a shellac finish is the fact new shellac applied will partially dissolve the old finish and smooth small problems.

However, adding shellac over a deeper gouge may not really help. Try wiping those areas with denatured alcohol and see what difference that makes. Sanding before recoating might be necessary.

Because shellac behave's just as Mac says,it is the simplest of all finishes to repair. It require's experience but even scratches deeper than finish can be repaired useing special heated tool to melt colored sticks into scratch. You are on the right path with flakes. Flakes will store for years whereas premix has a short shelf life. Keep different color flakes on hand.

In addition to the different natural color shellac flakes you can easily color shellac to any color you need using alcohol-soluble TransTint dyes.