Re: Spraying bursts
We have used water based colors applied directly to bare wood for some instruments. It will give nice fading for bursts and avoid the hard color line often seen in bursts. The downside is that the color is in the wood, and it will not come out readily if you make a mistake. You could practice on a piece of wood before applying to the instrument to see you are getting the burst you want. Then when the color dries you can put your varnish on top.
You can also put a thin sealer layer on the wood and then apply your color on top of that. If you use a poly sanding sealer as a base coat it should not be affected by the Epiphanes. It would allow you to strip the color if you were not satisfied with the results.
A very thin coat of nitro under the color would also work. Just be sure it is sufficiently dry before adding the color. If you use a thin coat you should be ok. If the finish is dry completely before applying the varnish it should be fine. You can use a French Polish on top with tru oil as well.
The color will work differently on the wood depending upon whether you apply it directly to the wood or on top of a base coat of finish. A good example is the original Loar mandolins. The burst seems to blend from shade to shade very nice. On some mandolins where they use an undercoat you will often see a more dramatic change in colors. Neither is wrong or necessarily bad, just different.
Have a Great Day!