You could always give the headstock a wipe over with metho or thinners to get rid of any oil residue, then when dry a very, very thin test coat of nitro to seal it. check for any reaction. If not leave for a day then another very thin coat of nitro. let dry then sand with 600 grit very very lightly. Put about 4-5 thin coats on if no reaction, lightly sanding between coats. Then try decal when very dry, about 3-4 days, again with very thin coats. the orange peel effect is usually from a too heavy coat.
Hope this helps.