All that's really needed on the original paint is for it to be sanded
till all the peel and shine is gone.
you can get enough adhesion by just scuffing it but sanding will
give a flatter base to paint over. 400 to 600 works pretty good.
Air sanders work pretty good but it's to easy to break through.
They're to agressive, you have to be careful.
I believe hand sanding always does better.