You need to use a wax remover before you begin sanding, or you will heat the wax and melt it into the existing surface.
After sanding, make sure you get all the dust off the surface to ensure a good adhesion bvetween the surface and your new paint.
If you're using a good, thick paint, you probably don't need to worry about spider cracks. If you do want to address the stress cracks, you have to grind them out with a dremmel and fill them with resin.
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