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?

go back to bare! its great stuff when used correctly. going over primered rust is not the right way

Is it a grey or red primer???

It's porous...

Per the primer, you would be degrading the primer with the acid in the ospho and it may affect the color coat adhesion or disintegrate the primer under the color coat.

Use self etching or epoxy primer as a base coat. They are NOT porous. Then you can add primer surfacer later.....

You need to take it back to bare metal and treat the bare metal.

Bare metal - self etching primer

I thought Ospho did the etching????

OSPHO IS PHOSPHORIC ACID !!! You do the science. The stuff is made for bare metal only adn should be put on bare metal ONLY !!!! You used a cheap primer that does not etch and you either need to strip it and apply an etching primer or spot strip it, re-prime those spots and get a good sealer on it. You get what you pay for IF you do use Ospho FOLLOW THE FREEKIN DIRECTIONS. It is not a brush on and paint over product. It WILL cause you problems if not properly neutralized.

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.

Strip all paint, primer, and other contaminents away before using Ospho! The stuff rocks, but it needs to work on rust/metal only, not a bunch of other surface crap.

I wouldn't be in here asking for help if the directions had this information on it now would I?

I read the bottle's directions 5 times, trying to make sure I use it properly. I AM SORRY IF THEY CAN'T ANSWER EVERY QUESTION ON A QUART BOTTLE!!

Geez.... who pissed in your Wheaties this morning.