Using plastic filler over primer?

[BIG D]

Can any one tell me, when you have replaced a panel ie front arch and you have used wax/silicone remover then used a rust preventer placed panel in spot welded into place, then covered area in primer to prevent rust etc can you just use filler or does the part that you need to fill need to be sanded back to bare metal?many thanks dave

[Titus A Duxass]

Filler can go over primer, just rub the area down with some 250 grade wet and dry first.

It is better to apply it to bear metal with a rough key (use 80 grit) and build on top of it.

[BIG D]

thanks for the info mate