You would normally put some coats of Tru-oil on first, then put the decals on, then more coats of Tru-oil over the top. So you can start to put the Tru-oil on, but I wouldn't put more than 4 coats on the headstock before adding the decals. You'll want several coats of Tru-oil over the decals to protect them and have enough depth to sand the Tru-oil flat then polish it once it's cured.