Hi ,
routing of X FG should include operation with control Key PP02 which will trigger External operation Purchase Req upon release of Production order.
Job work charges you mentioned would have to go as Purchase info record for vendor doing coating work. Purchase infor records should be of 0 type- Normal( Not subcontracting) without Material number.
for PP02 control key system will ask for Cost element to post cost directly as expencse ( To GL). You also have to mention Purchase org.
SAP handles Production subcontracting diffetrently then normal subocntracting (Like you have for part C made from B)
This is because you need to create many part number and breakdown BOM for just doing one operation at vendor place
Hope this helps
Jatin