How is the metal underneith it?
If the metal is smooth, spray on enough primer so you can block sand (I usually spray two coats). Block sanding requires you to put on some kind of guide coat (use guide coating or some old contrasting spray color) and sand it off with 180 or 320 grit sand paper. If you start with 180 sand again with 320 to get rid of the 180 scratches. You block sand until all the guide coat is gone. What guid coat color remains will show the low spots. Run your hand over the panel to show the high spots - your hand is a good judge for waves or high/low spots.
I recommend a two part primer. The dupli color or some part store primer isn't enough to get a good build. I hope this helps.
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