Hi poplars, It looks like it might be the sight of a dent which has been knocked out and painted over without taking the finish back to bare metal. The problem is that the paint did not stretch with the panel so has resulted in the lower layers of paint cracking. The only way to deal with it is to take a reasonable area around the damage back to bare metal, apply an etch primer suitable for aluminium and refinish with high build primer and top coat. You need to make sure it is not painted in synthetic if you are putting cellulose on top as it will pickle and react. The site of the dent looks to have had a secondary paint finish to that of the rest of the roof and obviously done by a home restorer so I would get all of that area removed and start from bare metal. Be patient and let each coat dry for a good while before the next observing flash times. Before doing anything use a panel degreasing solvent to clean a large area around the repair as you don't want to rub contaminants into the lower levels of paint and use it after prepping the panel. Most paint reactions are caused by insufficient flash times and environmental factors like humidity. Only paint if humidity is less than 60% as moisture will turn the top coat milky and dull otherwise.
I would tackle the paint by rubbing down the area to be painted with 600 grit, etch prime, lightly rub down with 800 grit, high build prime, 800 grit to give very flat finish and top coat 3-4 good wet coats feathered out towards the rest of the roof. After a good drying time, several weeks if it is cellulose, go over repair lightly with 1200 grit and polish with compound but take your time and be gentle. If you use cellulose don't use a cover on the car for 3months or so as the solvents will still be evaporating and it will be soft. If you want to save yourself the trouble of what can be rewarding and frustrating to a first time automotive painter I would get a quote for smart repair/bodyshop to do the work. They will typically use the latest paint systems which are far superior and are likely to get a better match, and as the rest of your car looks so stunning you probably want a near invisible repair which will do it justice.
Looking at your PIC the whole roof(certainly ALL of the roof that's visible)has problems.Looks like a very amateurish attempt to cover many layers of old paint.You are virtually guaranteed to get recurring problems by adding even more primer/paint etc.Is it just the rear aluminium part of the roof?If it was my car I would strip it all back to bare metal and start again with fresh paint.That way you can be certain you have potential removed future grief.
its the classic sign of too many/too thick layers of paint,you will never cure it by flatting and repainting it will come back. its caused by the metal expanding and contracting with the sun,but the paint dosent ,so it cracks. The only cure is a complete strip back to metal,and re paint etc. If you are lucky,very,sometimes with a orbital sander you can take the paint off ,back to the original primer,and as long as that is sound,but usually the cracks have also effected down to the primer as well,you can just top coat.Good luck .
Thanks a all. No it's not all, the roof, I zoomed in very close In the bad area so it could be seen. May well be beyond me to rectify so likely to look for a decent repair place in Lincs area. I know I can live with it for a while...but will annoy me. All advice very helpful .
