The Interprotect has ceramic platelets that make it tough to sand. Even if you can smooth it, applying another coat will bring back the orange peel if you use a roller. You will not have any more luck sanding it after it has set for one or two days than if it had set for 1 or 2 years. The stuff is just hard. The only way to apply it smooth is by spray, and you don't want to do that outdoors in a boatyard with other boats around, unless you can tent it. Even then, you need to start with s smooth surface if you want to end with a smooth surface.

I think if you really want to get it level, stop sanding the Interprotect and use a power washer or walnut shell blaster or soda blaster to get the ablative off. Then use a filler and sand the filler, then paint again.