I started off about 4hrs ago sanding back a patch on the roof of the L series about the size of 2 x 50c pieces. There was a bit of surface rust but nothing much else. I feathered the edges, cleaned things down properly and applied a light coat of primer filler. As this was drying the original paint started to lift on the edges of where the damaged section finished and the original paint started. I let things dry and sanded the original/lifted paint off again ready for more primer filler. Same thing happened even though, as before I had only applied a light coat. Sanded things back again and tried only etch primer. Same thing again. by now the area I was spraying had tured in something the size of a bill, ie, 10, 20, 50 etc dollar bill which is way out of hand from where things started. The primer filler, etch primer and paint are all for acrylic paints so what am I missing ?

When the original paint is lifting off it is only going back to the original undercoat which I guess is a good thing as I am not going back to bare metal. Any thoughts on what is causing this ?
I am now up to the stage of applying the white acrylic paint very slowly and in very light coats and if the edges lift slighty I am lightly sanding this back, letting it dry and putting more white on.
While I hope I have sorted this spot out there is a fair bit more to be done on the roof such on top of the windscreen and above the gutters in a couple of places.