Im Restoring a fender and i have cleaned it well sanded it clean and ethed the metal
Question----------
What is best to use so i can hold rust while i make repairs to some minor dings then paint?

Metal Prep. It's available in the paint department of home centers or hardware stores. It's phosphoric acid and will slightly etch bare metal and leave a paintable film. As long as you don't leave the metal out in the rain, it stays put. If you handle the fender a lot, you may have to re-treat before painting.

I agree. I used an automotive etching/filler primer prior to doing any bodywork. The nice thing is is that you can always touch up spots after doing the bady work. I did however use a wax/grease remover and then a tack cloth prior to painting.