PetroStreet

We have fabricated a pretty long piping designed against ASME B31.3 and hydrostatic testing has been carried out in sections.
Now the piping has been installed in the field and a final hydrostatic testing is required to be carried out. Is it necessary to keep already tested weld joints uninsulated also or just those which have not been tested before?

No. Not required as per ASME B31.3 para 345.3.1.
You can insulate the joints which have been previously tested.

Thanks for the quick response.