Yea its deffo a twat to do.
I scraped it all out using a long screwdriver and various bits of metal I had laying around (sticks etc

)
I then hoovered it all out, the best way I found of doing this was to cut a piece of the old mans hose pipe and cut an angled end on it. Now feed this into the end of the hoover flexible section and through the smallest attachment. Stick you thumb over any bit left and stick the other end into the roof.
To treat the rust I knicked two of those long poles which hold plants up from the garden and zip tied a brush to the end. This was then used to spread the 'krust' on all the rust.
I bought a high-pressure waxoil kit and just sprayed to my hearts content

I put loads of the stuff in there so hopefully that should stop it.
Oh and in the summer the car doesn't fill up with flakes of rust everytime I open the roof
Hope this helps