I've used RnB for years and found that the best way to apply it is on a "dulled" or flat painted surface first. Rubbing on a smooth glossy surface doesn't work as well. The previous comments are correct about sealing with a clear coat...it will remove the metallic sheen. Depending on how thick your spraying the coats, it can even cause the RnB to run and streak. In some cases, that actually creates a interesting rustic appearance but requires another RnB layer for ultimate realism.
A good trick is to use a blow dryer to warm up the RnB after applying (be careful not to hold the dryer in one place too long) and then quickly cooled it down. That seemed to accelerate the bonding and minimized the flake removal most of the time. One process to try out...apply some RnB, let dry in a warm environment for a day, then spray on a dull coat, let dry for at least 8 hours and reapply the RnB...finally, use the blow dryer as mentioned above. This has provided me with great results and it adds a subtle realistic touch.