Wipe On Poly (either pre-made or simply diluting your own) is usually described as the closest thing to a fool-proof finish as you can get (as a finish by itself). I might argue that an oil-varnish blend might be even easier.

But yes, oil-based poly will yellow over white and if you are applying it over another finish, like paint, it might not adhere very well.

But all this sort of begs the question. Paint is essentially a clear finish (lacquer, alkyd (oil-based) varnish, or acrylic (water-borne) finish) with pigment added. Why would adding more of the same, without the color, significantly add "protection?"