So i have been removing rust from my vehicle and on the quarter panel and the door there are some spots that after the surface rust is removed are dark spots and tiny pin holes in the metal

In a few areas i notice that after grinding with a wire wheel that some holes went all the way through the metal, i am using ospho to treat it and i wasnt sure if i should continue grinding as the metal is getting pretty thin

I am quite concerned about the rust that has apparently been aggressive enough to make tiny holes in the metal and go all the way through to the other side


FYI: The reason there is rust is because i had my car improperly worked on and the people put a fiberglass wide kit on it and used bondo which did not seal the car and when i decided to remove the kit due to cracks and redo it i am noticing all the rust in those particular areas

Photos attached

You need to media blast areas like that if you hope to salvage them. Sanding, grinding, wire brushes will not do the job and like you found out remove too much metal.
Looks like you will be welding in some patch panels as well. You can read up on that process in the Welding and Metal Fabrication section.

With the current condition of the metal, would por 15 work, im not concerned about the paintability or looks of those areas as a fiberglass panel will be going over top of it

If not i guess ill look at the welding section

I am not a fan of Por15.

You can treat the rust with Ospho. Once it has turned the rust black you will need to apply another coat and then wash it off with soap and water before the Ospho dries. This is a necessary step that neutralizes the acid so it won't come back to bite you. Ignore the label that says the treated metal is ready to paint because it's not. The rust treatments leave a film behind if not properly neutralized. (see pictures of the 1962 chevy truck I am stripping now)

Once neutralized and rinsed well with water blow everything dry with compressed air. At this point I would spray or brush on a quality epoxy primer. You will want two coats and let it cure at least 24 hours if you plan to prime and paint over it.
The epoxy will protect the metal from rust and as long as it is not exposed to direct sunlight (UV rays) it will be fine just as it is.

Oops, on some other areas i applied ospho and left it on i thought it was a protective layer


I have ********** primer so i will use that, i have a concern that the rust has penetrated to the other side so will the ospho and epoxy primer prevent it from spreading even if i dont put primer on the other side of the metal

You need to reapply the Ospho and neutralize as stated above wherever you plan to apply primer and paint.
The back side of panels can be left as treated with Ospho since you will not be applying anything over it.
As to the longevity of the rusted metal after being treated I cannot say for certain. I used the Ospho on my horse trailer frame, neutralized and then brushed on 2 coats of ********** black epoxy and it has been several years now and it look great.

Appreciate the info

I meant the backside cannot be reached so i wouldnt be putting ospho or primer on it, and since the rust formed pinholes i assume the rust will spread from the backside now or will the ospho primer treatment prevent that

For example the rust will no longer spread towards the treated and primed area but would it start to spread and come out the backside of the metal, or has the rust been completely neutralized

Another thing is i am worried i may miss a spec of rust while treating it, and that will continue to spread

Without getting the Ospho applied to the rust on the backside of the panel you will have rust eating its way from the inside out.
I have put some in a spray bottle to squirt through access holes but WARNING the vapors are killer and you will need a good respirator and ventilation.