No great secret Marg ,I just use good quality brushes & the process goes. Sand well I go down to 8oo grit paper,Undercoat,Lightly sand again, Coat of undiluted acrylic paint, Sand again, Final coat of water diluted acrylic paint, If I varnish the piece I thin the varnish with Penatrol for that smooth finish.My segmented pictures I now do with an airbrush as it is much easier & quicker on those small pieces & gives a better finish,I still use a brush to apply the varnish.Its all a matter of good equipment & patience,Hope this helps you in some way.
Cheers Peter.