This is used as a white base for best coverage for the colour coat. It helps identify anything missed in the grain filling/sealing process. Any imperfections can be fixed before the colour and clear coats. A colour coat will bond (melt) with the sealer easily due to its nitrocellulose properties, but the "primer" was usually used for vintage instruments after the filling/sealing process. It goes on fairly light and will opaque more and more with each pass you make. Apply until you achieve good coverage.