2 Piece pretty much hit the mark on primers and sealers. The discussion has mostly been about Lustercote but I find that these observations are true for other types of paint when you are trying to lay down a coat of yellow. Most of the yellow paints I have run across behave like a clear coat in that their performance is influenced by what lies beneath. Both yellow Lustercote and yellow dope are really pretty transparent and need a base coat in the same manner as a two stage automotive paint. BTW I would test a sample before trying to mix paint types. The appearance of the yellow Lustercoat is strongly influenced by whatever is beneath it. As 2 piece notes the Lustercote primer isn't a particularly good sealer but the coats that you have put on the bare wood should have largely sealed it. One thing the Lustercote primer does is provide a consistent base color so that your yellow will come out consistent. Depending on the paint type, colors from the yellow to red range will be effected by the underlying surface. Automotive painters use different base coats to get a desired effect.The short version is you will need a consistent primer across the aircraft to cover up the underlying structure and to get a consistent shade of yellow.