Question About Sanding

Even if you get all the rust thats visible you wont get all of it.What you need is some kind of converter.Best yet coat the area with a bare metal primer that has zinc chromate in it.Any kind of primer for metal and a marine application will have high corrosion resistance.
as for the holes,yes they need to welded or the metal has to be replaced.Once theres a hole the metal around it is pretty thin.Any sort of mig weldig will likely make the hole bigger.Too much heat and too thin a metal means the welders heat will just burn a hole.A better solution for pitting and small holes is brazing or lead.Lower heat and better at filling.Takes practice though.

If you need to replace metal and you have a hard area with compound curves,do it in several pieces.Cut one section out and replace it.Keep it small.Repeat until all rust is gone.Do it like this and your patches will be very close to original.

Blasting is an excellent way to get things really clean.Dont use sand.Use ground glass.It cuts MUCH faster and thus less heat and warping.