First, get the sucker clean. TSP, soap and water, etc. A good
detergent. Then sand lightly to "break" the current finish. Fill in any
dings, etc. Get the surface nice and smooth.
As to primers, you are likely going over an oil paint, so see what is
around for just that purpose, assuming you are going for a modern latex
paint. I've used outside semi-gloss paint as a primer, as it has lots
of pigment, and bonds well. A coat of that, and then a couple of coats
of your finish gloss, or semi-gloss enamel. You might be quite amazed
how much it is going to take to kill that purple paint! There is also
a wall primer for going over colored walls called KILZ, I believe, See
if they have a primer that would meet your needs.
Lowes or Home Depot are your best bests. I might even consider two
coats of primer, and some light sanding between coats to smooth things
up. Make sure things are DRY before you put on the next coat. At least
overnight, and longer if you are working in the garage where it is cold.
A white, oil based primer will work the best. But since you will most
likely have way more primer than you need (no matter what kind you buy)
you can always coat it enough times until you can't see through it.
Better yet would be to use a heat gun to remove the old paint, then
sand, then prime and paint.
Try a low-voc transitional primer such a AFM's so you can transition
from the old stuff to a nice new surface that you can paint with a good
no-voc paint - such as ICI's or AFM's.
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