Sonus was once one of the more popular brands used, and while it has fallen out of favor slightly, it has still has a loyal following and for good reason -the products still produce the same tremendous results!
SFX1 is a fairly aggressive polish that is targeted at restoring heavy defects and oxidation from weathered paints or deeper marks from hard paints. Whether you need to use it or not will depend on the condition of the paint and hits hardness. It is good to have a more aggressive "cutting" polish like SFX1 around in case you need it.