Read the directions...
Every paint is different, and many will fail totally if the directions are not followed scrupulously. Sanding down to gelcoat/fiberglass is probably a good idea to avoid contamination and compatability problems with the old paint. Is Interdeck a two part polyurethane? We found that one-part polyurethane paint simply didn't hold up well on our decks. (A test patch lasted about a week...) Nonskid particles are generally added to most deck paints because they are too thin to create a rough surface by using a sponge. I am not familiar with Interdeck, however. Can you contact International Paint (their website or email them) to get answers directly from them? That would be the best way to avoid making any expensive mistakes. After doing it once, you do not want to have to repaint your deck any time soon.