Turning one’s rida’ inside out should be done during the khutbah of istisqa’ when the imam turns his rida’ inside out.
Question

                In one of the mosques I saw one of the seekers of knowledge wearing his ‘abayah (cloak) inside before the prayer for rain (istisqa’), and after the prayer I asked him he reason for that, because what I know is that the ‘abayah should be turned inside out after the prayer and khutbah are over. He said: There’s nothing with that, rather we turned it inside out before the prayer so that when we changed it again it would be right side out. What is your opinion concerning this? May Allaah reward you with good.
Praise be to Allah.
    The apparent meaning of the ahaadeeth narrated from the 
    Prophet  (peace and blessings of Allaah be upon him) concerning the 
    prayer for rain (salaat al-istisqa’) is that the rida’ (cloak, upper 
    garment) should be worn as normal, and it should be turned inside out during 
    the khutbah when the imam turns his rida’ inside out. 

    With regard to turning the rida’ or ‘abayah inside out before 
    that, the more correct view is that this is not prescribed and is contrary 
    to the Sunnah. May Allaah help us all.