The paint is not a pro finish obviously but i treated, primed and coated and clear-coated it and the fenders and boot for example don't seem to have this issue but the roof gets these blue-grey streaky rusting marks in places. Am i just not treading the rust right or is it something else i am doing sloppy on? Any paint-rust experts about?

 

All that lot needs to come off to bare steel, and the rust cured with a rust dissolver like Jenolite or similar. Then prime with zinc-rich primer, followed by normal primer, then your topcoats, (base and clear).
I have to say the standard of finish you've got there is pretty poor, awful even ! Might be good to take advice from a person who paints cars for a living. Where are you painting the car ? You need to be in a totally dry place.

 

I agree with Fraser. There is no simple cure for this sort of rust. It will have to go back to bare metal and be properly treated with a phosphoric acid based rust converter. Then follow Frasers recommended steps.
Trying to manage/ paint over this current mess will not work.

Bill Mac
MK1
MK2
XJ6S3
X300
Previous 14 Jags MK5 to X308

 

The other thing I forgot to mention is that removing the rust will leave pitting in the steel where the rust ate into it. ONce you've got your primer on, you'll need to fill the pitting with stopper. If it were me, I'd leave it till you have the final primer coat on. You then carefully flat off the stopper to give a smooth surface, then pop another coat on., flat off again, then colour coats. It'll be perfect then.

 

i see it is complex.
the cracked paint was striped off to the steel about two years ago and treated with a rust killer, the primed, and a top coat applied as a 'holding coat' for the time being as car immobile. idea was just to protect the steel. this worked for the fenders, bonnet, boot and a few other parts that could be removed and worked on indoors. the roof of course could not, wokred on outside, so maybe the steel or primer accrued some moisture or the rust killer zinc stuff didnt do it job perhaps. seemed all clear at first. just broken out in these 'rashes' while the parts worked on indoors have not done so.

 

Painting somthing like this outdoors is almost guaranteeing failure of the paint, IMHO. Once the steel is back to bare metal and de-rusted, it needs to be protected very quickly. Even if it seems perfectly dry and warm, overnight moisture will fall onto the bare steel. There are primers, and primer-surfacers, but all of them can absorb moisture, if left before proper top-coats are put on. My advice is to get the car inside somewhere to paint it, or hand it over to a car paintshop. Yes, it'll be expensive, but paying for expertise and indoor premises usually is.