Rusty Roof

[Lost81c10]

I am in the process attacking something that is or has been all too familiar to all of us, fixing the rusty roof on my 85 as it really needed the attention before it becomes too late. At first look I thought it had some surface rust and thought that I would hit it with some 80g and it would come off in no time however, I quickly realized that it would not be the case. The sanding did work to a degree, but the rust was still there. So I had to turn to a different approach, (chemical approach). I have been using the Naval Jelly approach as stated in a thread in another forum which some of you here were involved in and it has been working for the most part as you can see in the first pic, but it has been a time consuming task with plenty of elbow grease to go with that. On the first pic I have done about five applications and after each application process, I hit it with 80g before the next.
My questions are; how much of the rust does need to come off before it can be safe to shoot epoxy? Does it all need to come off? If I can get away with not all of it coming off, do I have to do other prep before epoxy? Does anyone have an easier tried-and-true approach you might have used, I guess blasting would be a lot easier but I don’t have that kind of eqpt. I am willing to continue this process as it does work, slowly, but I just wanted to ask before I continue with it. Thanks for the help.

Mando.