By sanding and alot elbow grease
I stared with Grit P180 (normaly applied to wood, but gets rid of the paint)
Grit P320
Grit P800
Grit P1000
Grit P1200
Grit P1500
And at the end, i used "autosol" to polish the surface with, this product is available in most countries
When you are done the surface should be very smooth before you apply autosol
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