Just let you know eps is the core not epoxy core, epoxy is just a type of resin that you lam with the fg on the outside. But anyway, If you didnt lost too much foam from cutting away the delam piece of fg, i would just use epoxy mix with microballons and it will turn into a white paste, after it cure, sand it smooth and just do a step patch with one smal size than other. probably 2-3 layer of 6oz fg is good. prime and paint it. It will be really hard to match white color so you can be creative with other colors and mask some pattern on it so the repair wont be too visible.
Can I sanding the entire board with fine grit like 400 and use some automotive spray paint to add some color(I think yellow), after that sanding with 1000 grit wet and apply a final hot coat over with epoxy/aditive F?
i think epoxy won’t bond well with autopaint. it is best if you just fix it normally with epoxy/fiberglass and if you want to add color, just spray the primer/paint after you hot coat and sand it smooth. That is how your board was constructed, it all paint on the outside and glass and foam in the inside. If i were you, i wouldn’t even paint it since it will be cover by the deck pad and it is a cheap SUP don’t waste time and $ on it. fix it, ride it and sell it when you are done with it.
Use a good quality marine paint there are some excellent mono epoxy paints and some marine acrylics That give a very nice durable finish. Any good boat supply store should have a primer and paint that would work for your project.
I did a similar repair to a Doyle 12’ SUP on the bottom, be sure to flare in at least an inch past your cutout to bond to existing board laminate that is still sound. When you sand back try to get an even bevel exposing the 2 or 3 plies to the paint. Then laminate you new glass to the same sizes and put an extra layer capping the whole repair. Now blend the repair to the original laminate and fill will 2-part epoxy filler to get the surface free of pin holes before you paint. Since most of the area is under the deck pad you can get away with a less than perfect repair. For a $100 it was worth the foam alone! Good luck and have fun paddling!