Are your interior units currently the original gelcoat or have they been painted?
If they are gelcoat, I would suggest first de-waxing with something such as Interlux 202 (the instructions will be on the can; it's very important to use plenty of clean towels so that a used section of towel does not re-swipe the gelcoat). This step should be done before sanding so that you don't just sand the contamination in (I know it seems weird, but the mold-release wax will easily last 30+ years - that's what you are removing).
Once you have the surfaces de-waxed, then I would sand with whatever grit your chosen primer calls for. Something in the 120-220 range would be typical, but its best to follow the paint/primer manufacturer's instructions. This is also the time to fill any imperfections. Also, I would not count on the primer hiding any flaws; in other words, get the surface as perfect as you want it, by filling and sanding, before you start painting