If you're looking for a way to tell which items are still in your warehouse vs. which are out at the vendor, there are a couple of ways that I explain to customers some require more steps than others, so I let the client decide which will work best for them.
1.Create an ""outside processing"" Warehouse
a.Process a warehouse transfer to that move the items to the vendor warehouse when items are sent out; if you only have a few vendors, you could even setup a separate warehouse for each vendor.
b.When the items come back from the vendor:
i.Set the work order to pull that item from the vendor warehouse and do the material issue as you normally would to issue them to the work order.
2.Setup two different item codes for that item ""WIDGET"" and ""COATED WIDGET"" for example then do the following
a.Create a separate work order with parent item COATED WIDGET
b.Put the WIDGET on as the component item.
c. When you send the item out to the vendor, issue the WIDGET quantity to the work order using a material issue transaction. This takes them out of your warehouse; you can look at these work orders to see how many are issued.
d.Use the method Doug described to apply the cost of the powder coating when the items are returned to your facility
e.Complete the work order for the number that you have received this will put the finished items back into inventory under the new part number with the item cost plus the cost of the powder coating.
3.If bill of materials is used, but not work order you can use a combination of 1 and 2 above.
a.Create a BOM for the original item (WIDGET) and a miscellaneous charge for the added cost of powder coating
b.Transfer the items to a separate warehouse when they go out to the vendor
c .When the items come back from the powder coating vendor:
i.Create a production entry set component warehouse to the vendor warehouse,
ii.Produce X number of COATED WIDGET parts. This will pull the WIDGET parts from the vendor warehouse and place COATED WIDGETs into your warehouse.
iii.Enter you vendor invoice using the same G/L account that you assigned to the Miscellaneous cost on the BOM.