Painting The Walls - Flood Repairs.

Hi I'm looking for some advice from the painters out there. Labour costs have gone up since the floods so I'm intending to do some of the exterior painting myself. I know the usual procedure for bare concrete would be sealer / primer / then the paint, but what if the walls are already painted? Can I sand is a little to make it smooth then put the paint straight on, or is the primer also needed?

If I use filler, which I will need as some paint has already come off to the concrete, do I put on the sealer, followed by filler, sand it, then primer then paint?

If anyone has advice I'd be glad to hear from you.

Any what is the best brand of exterior paint to use here in Thailand?

Thanks..