Not necessary. Providing of course that the original finish has no major scars, runs in the paint, "orange peel" or something along those lines that would show up underneath your new paint, then sanding is not necessary. And besides, people put too much emphasis on perfectly smooth tank paint that they forget that the paint finish was often far from perfect and the armor itself was very rough in texture. Primer is not necessary unless you sprayed a much lighter paint over dark which could require that you make as many as 5-10 coats to cover up the dark underlying paint.

If you're doing a winter whitewash on a Tiger however, some see-through effect showing the undercoat would look best.

Which tank have you decided on?

-Harq