Whats the decal printed on to?

If "easy apply air release" then they should be done dry. ( can tell as vinyl will be made up of lots of hexagonal shapes that press out when applied)
If going onto bare timber then decals should be on a hi tac vinyl.
If in any doubt, substrate should be sealed with a smooth finish... not acrylic wall paint as this is absorbant and generally long term adhesion cannot be guaranteed