I agree with Dave.
I’d give it a taste of 60 or even 40 grit to clean up that odd patch to the left of the mouth and the area in front, and then use the plane. If you use 40, use some 80 and 120 afterwards, and smooth the edges of the plane at the end.
You don’t need to get all the marks out to make a working plane.
Also, are you sanding it fully assembled? The plane needs to be fully assembled as the pressure might distort the shape of the body, and you need to flatten it like that.