Ospho over primer? Options

[KaptKaos]

I have some surface rust peeking through some spots that I have primered. What happens if I brush some ospho on those spots? Will it seal?

Or, am I better off just going down to bare metal and applying it?

[charliew]

All the above. But I like to go ahead and put the sanding primer over the non sanding epoxy so that it will be ready to start the sanding and filling process if it's going to be a long term process. Epoxy primers are meant to be used only as a base primer. They do seal very good but they are not meant to be sanded later and then the sanding primer applied. If the epoxy primer is sanded then more epoxy primer should be put on before the sanding primer is applied. If I'm going to immediately start using filler I will put that ontop of the epoxy primer then after leveling use more epoxy primer to seal the filler up and then use the sanding primer to get the scratches out. Epoxy primer is not meant to set more than about 12 hours without adding the sanding primer or more epoxy must be put on after it is sanded or scuffed which is just more work.