There is quite a lot involved in painting such a large panel.
The roof is on the eye line so needs to be a respectable finish.
What grade paper did you use?
Did you lubricate with water?
Have you got tac-rags?
What will you spray with?
Do you have a dry wind free ventilated environment?
And a baking oven?
Or is it to be cans & sod the finish as long as the colour matches, sort of thing?