My house is 12 years old and the paint on the outside concrete walls has held up very well. Only thing is, it has become dull, so I would like to repaint. I have read all the threads about painting new surfaces but have not seen anything about if primer is nessesary when repainting.

The surface is very sound, and of course I would like to simply apply paint only, but if I need to, I would rather do it right and prime also. Can someone please advise me?

I would follow the instruction on the can of paint you propose to use as the final coat. Often it will say something like "remove loose paint and prime bear areas"...or something like that.

Chownah

I would strongly recommend applying an undercoat first. It is indeed the safest and professional option.

If you wipe your clean hand across the wall and your hand has a chalky powder left on it , remove loose and flaking paint, Stabilise the walls (one coat) then give two coats of decent grade Exterior Emulsion.