Shiva,
Check in the EDI 810 dictionary/Schema of that particular segment/field properties whether it is defined as 2spaces.If so you can edit the dictionary/Schema (and migrate the same stuff when you move to different environments) or see if any fix available in Advantage website or contact tech support as a last option for fast resolution.
Since convertToString has nothing to do with this as it is structured based on the EDIFFSchema.
HTH,
RMG.
#Integration-Server-and-ESB#edi#webMethods