Filler over light rust

[drgoatboy]

Sounds like a daft question but if you've done some welding on some panels and it's not got filled straight away and the bare metal has got some light surface rust is it ok to just filler over the top of the rust?
Would the moisture barrier created by the filler be enough to stop it rusting? Or is this just going to keep rusting and eventually ruin the panel?
Thanks!

[drgoatboy]

Ok thanks. What's best to do then. Grind down then fill?