Shall, Will and Must, in everyday usage are used with varying degrees of interchange, whether correctly is another matter.  In writing contracts, or at least the ones that I was involved in, it was quite important to use the right word of the three. 

Before I retired I was involved in quite a few projects  of varying values up to multimillions £ and as the client's representative on these projects it was vital that the correct words were used.  Whenever one wanted to ensure that the contractor was contractually liable to do what you wanted him to do one always used 'shall' and never 'will' or must'.  So for example one would write that 'the contractor shall repaint all the pipework every five years' not that he 'will' or must'.  However, if the contract referred to what the client was going to do then one always used 'will' as in 'OPA [the organisation I worked for] will provide the contractor with a complete set of drawings within 28 days of the start of the contract'.  This meant that OPA was not contractually bound to do what was stated in the contract in the way that the contractor was.  Must, however, was never be used in contracts, or at least it was never used if the person writing it knew what he was doing.

Tim

Tim,

 Though English is not my first language I think I wonder about using the word "shall" instead of "must"

in the sentence you use about repainting the pipe working.

He shall repaint, yes it is clear enough but he could get away with it if unforeseen circumstances would occur BUT he must repaint indicates to me that whatever happens he has no other option and is obliged to repaint the pipework.

Tim, I might be totally wrong on my interpretation of shall and must but that is how I would read the clause.