Washing a flat surface is a very difficult thing to attempt because the wash will travel through and across it quickly. You should first seal the paint with a gloss coat like Future or other clear coating, let that cure for two days then apply your decals, another coat of clear then the washes. After the washes have dried put on a flat coating like Polly Scale clear flat or Testors Dull coat to bring the finish back to a flat look.
Doing it this way will protect your base coats, prevent your decals from silvering or peeling and result in a nice finish.