Clive, remember that what you are doing is inducing the Sabattier effect. You are not Solarizing the film.
With Sabattier, you superimpose a positive image on a negative image during processing by means of a short flash. Solarization is gotten by very very long exposures in-camera.
Also, Solarization is very uncommon with paper, as it requires a camera exposure and it uses very long exposures due to the low paper speeds.
They look quite different.