Don't know about psoriasis but I have skin that is highly reactive and subject to dermatitus breakouts. (Irish celtic ancestry. I should be living in a cold, damp climate not the dryest continent on the planet.)

A dermatologist here in Sydney once told me to use nothing but sorbolene because, being chemically inert, it will not cause any skin reactions. Get the pure kind with 10% glycerin and no additives. 

Use it  for showering/washing. Then, when you get out of the shower and your skin is still damp, smear a thin layer of sorbolene all over your skin. This provides a protective seal to prevent moisture loss from your skin and allows your skin's own oils to lubricate it.

I do this. and no skin problems.

Evelyn