First, you want to wash it thoroughly with Dirtex (strong solution). Second, sand it with 180 grit sanding sponge until it is dull. Remove sanding dust with vacuum and damp sponge.

No primer is required (unless making drastic color changes) when repainting clean, dull, alkyd paint with high quality acrylic enamels.

Thanks so much, brushworks.

If I wanted to avoid harsh chemicals, is there
an alternative product I could use to wash it?

dirtex is not really harsh