Morning Roy,
The product we recommend for grain filling prior to french polishing, or lacquering, is called Thixofilla. This is a high build, low odour, thixotropic grain filler that will enhance the grain of most timbers.
It comes in a range of wood shades including neutral/clear. If you have a rather dark timber it's better to go for a coloured Thixofilla in this case Mahogany. The neutral is better for lighter woods but can make darker timbers appear a bit lifeless or slightly grey.
Application is by lint free cloth rubbing in a circular motion and then rubbed off across the grain. Once it has been left overnight to dry it can then be lightly sanded prior to finishing with french polish or lacquer. It's available in 1L & 5L sizes and will cover approximately 3-5 sq.m/lt.
I must admit it makes such a difference to the final appearance. Any other questions please don't hesitate to ask.
Ian