so that upon expansion one portion of filter media 28 does not exhibit sliding movement with respect to another portion the opening size can be made to suit the significant feature of the filtering material 28 is that it is flexible thus when the string 20 is preformed into a corrugated shape as shown in fig3 by using known techniques such as pulling it through a die the filter material 28 can then be applied over it as shown in fig3 thereafter when the material 28 is properly positioned in the wellbore a known expansion tool illustrated schematically as 30 in fig1 can be inserted into the string 20 to take the initial shape shown in fig3 and expand the string 20 under the filter material 28 to a rounded shape as shown in fig2 as a result the filter material which is flexible expands with the underlying tubular 20 as the shape of tubular 20 changes from that of fig3 to that of fig2 a cover material 32 can overlay the filter material 28 for running in so as to protect the filter material 28 from gauges or cuts during run in the material can be a thin sheet which snaps upon the slightest expansion of the corrugated tubular 20 it can be a elastomeric material that literally rips at the slightest expansion of the underlying corrugated tubular 20 as shown in fig3 other materials for.