How can I paint old furniture that is dark to light it has schelac on

Sand it really well or use a chemical from the local hardware store to get the color/shine off of it.

Yes, those liquid sanders really do a good job getting into crevices and such. Also a good white primer for light colors (darker colored primer is great when you want to go dark or bold later on).

Give it a light sand to smooth, and prime, then paint.

I agree with Sharon. Just sand lightly to remove any gloss. Prime and paint. Seal with at least three coats of a water based polyurethane.

Use ESP on first then paint undercoat and then topcoat or use Chalk paint and Clear or Antique Wax to seal..