When you use SPI Epoxy primer there is no need to sand before putting filler down...Below is a quote from SPI's Web-site
On any restoration it is always best to apply the body filler over the epoxy rather than
bare metal for best adhesion and corrosion protection. If one coat of epoxy is used then
the body filler can be applied in 60 minutes. When applying two coats of epoxy, wait
over night before applying the body filler. The epoxy does not need to be sanded before
applying the body filler.
If your not using SPI, you will need to check your tech sheets.
Hope this helps