Which is the brush for paint, 100% China Bristle or Poly/Nylon? Are both types of brushes good for all the paints or some brushes are good for some paints or another?
Both types are used with different types of paint.
China (natural) bristle brushes are used in oil based primers, paints and wood finishes only. This includes alkyd primer and paint, varnish, polyurethane and wood stains.
Poly/Nylon (synthetic) bristle brushes are used in water based primers, paints and wood finishes. This includes latex or acrylic primer and paint, water based polyurethane, acrylic urethane and water based wood stains.
Many brush manufactures state "For All Finishes" or "For Oil or Latex", but the synthetic brushes don't work well in oil based finishes. Also, China bristle brushes can't be used in water based products, they will be ruined.