He did ghusl and prayed, then he realized that he had missed a part of his body and the water had not reached it
Question

                Is continuing the ghusl after it has been already done permissible? I mean if someone remembers after he finished ghusl that a part of his body was not washed, should he just wash this part, or repeat the whole ghusl knowing it is tiring?  

My case was that I had a sticker on a wound on my back. I removed the sticker and the glue that it leaves behind. Initially I thought that I removed it totally, and made ghusl. I realized some time afterwards that I did not remove the glue completely, knowing that the water passed on this part. What should I do? (I mean does the glue of a sticker stop the water touching the skin?).
Praise be to Allah.
    Firstly: 

    The fuqaha’ have stated that it is essential 
    to remove everything that may prevent the water from reaching the parts of 
    the body that must be washed in order for ghusl or wudoo’ to be valid, 
    because washing the parts of the body cannot be achieved otherwise. 

    Al-Nawawi said: If there is wax, dough, 
    henna and the like on one of his limbs, which prevents water from reaching 
    any part of it, then his tahaarah is not valid, whether the amount is large 
    or small. If there are traces of henna and its colour left on the hand, 
    without there being any solid material left, or elsewhere, or traces of 
    liquid grease whereby water flows over the limb but does not stay there, his 
    tahaarah is valid. End quote.  

    End quote from al-Mughni (1/529). 

    Secondly: 

    With regard to the ruling on the prayers you 
    offered when this glue was still on you, it depends. If the glue was thick 
    and prevented water from reaching the skin, then you should regard the 
    prayer as invalid and you have to repeat them. But if it was a small amount 
    and did not prevent the water from reaching the skin, then the prayer is 
    valid. You can determine that because you can see it, but if you are 
    uncertain, then to be on the safe side you may repeat the prayers. 

    Thirdly: 

    The scholars differed concerning the ruling 
    on sequence of washing in ghusl. There are two opinions: 

    
    1 – That washing the parts in sequence is not essential. This is the view of 
    most of the scholars as was stated in al-Mughni (1/220) where it is 
    the favoured view. Shaykh al-Islam also said that in Sharh al-‘Uddah 
    (1/208) and quoted a great deal of evidence for that, which you will find in 
    the answer to question no. 
    
    99543. 
    According to this view, if a person omits to wash a part of his body then 
    remembers it after that, it is sufficient for him to wash only the area that 
    he omitted, and he does not have to repeat ghusl.