Do you mean just take off the skin or talking about the core itself? Also if I grind fill what do I fill with? Some type of epoxy or just fiberglass resin? How do I go about smoothing it out? Sorry for the basic questions but once I get past them I should be alright. I think I will paint and sail and dig into it in 7 or so months. Also, is it possible to do this without taking the rudder off?
Judging from the patern of the cracks it's freeze damage because you have water inside your rudder. What causes me to pause is that you have rust dripping out; bad stuff.
First, tap on the rudder with a hammer of some sort and listen to the sound it makes. Dull or sounds hollow? If it sounds different as you approach the crack/blow out this means that the fiberglass/laminate has separated from the guts. (foam/wood/plywood)
If it sound hollow your better off taking the rudder off and following the directions from the link i provided earlier..
If it sounds good across the whole rudder proceed with the following.
Sand down the surface to about 8" around the hole in all directions. (60 grit)
Grind out/down the section that has all the cracks. No need to go all the way thru; close will work(1/16).
Taper away from center. 12:1.
Go to a West Systems epoxy dealer and talk to them.
Using fiberglass and epoxy; epoxy gradually larger circles of fiberglass over the hole/big dent. Three layers at a time. Wash surface and sand between layups. 5 layers will probably get you close.
Once you feel that it's good and strong you can "fair" your rudder with epoxy and light weight fairing compound. Just like do auto body work on a car except your using epoxy. Coat, wash, sand then coat wash, sand.
I am by no means an expert!!
Talk to your West Systems dealer. It's not cheap for the epoxy, pumps, spreaders, mixing bowls........ But worth it.
I'm sure someone on here will correct me on the procedure's.
Read this; it will help.
Now........ you will do all of this and it's a temporary repair! ... Next fall take your rudder off and either take it to a shop to have it repaired (which judging from the rust stains you may wish to do) or attempt to split and repair yourself.
Hope this helps