S.C. Code Regs. § § 117-309.6
Property manufactured or fabricated by machine shops and custom foundries is subject to the sales tax, except when sold for resale purposes or when exempted by one of the exemptions found in the sales and use tax law.
In doing repair work, the machine shop operator consumes the materials which lose their identity in the repairing process, such as paint, solder, babbitt and lumber. He is also considered to be the consumer of such items as cotter keys, nails, washers, stove bolts and nuts, bits of metal, and sheets of metal used in patching or reinforcing. The receipts from the use of these materials are not subject to the sales and use tax. The sales or use tax is due by the machinist at the time of purchase from his suppliers.
Where the machinist in making repairs, fabricates or manufactures a recognizable part or attachment for the article being repaired (as contrasted to patching, mending, or reinforcing weakened parts) no deduction is permissible for labor or any other expenses which are a part of fabricating or manufacturing the part or attachment. He may, however, if making separate agreements to sell the manufactured or fabricated part and to install same, remit tax only on the sales price of the fabricated part or attachment, provided his books and invoices show clearly a separation between the sales price of the fabricated part or attachment and the labor and service of installation.
S.C. Code Regs. § 117-309.6