All decals have a clear lacquer coating or binder and as the decals age this coating degrades to a point where the decal disintegrates when dipped in water , the solution is to recoat the decals with a clear spray of some kind . A clear lacquer such as Testors works the best , but just about any clear will work . Be careful with acrylic clear sprays like Krylon . While they dry very flexible which is a good thing , the decals also end up being easily stretched . With small decals this isn't a problem but with larger decals the stretching will actually crack the ink as you apply the decal ruining your day . Also be aware that decal solvents and setting solutions can dissolve Krylon if over used also dissolving the ink , so use them sparingly . Clear lacquers don't dissolve as easily and don't stretch .
Bottom line is you can use any spray clear but always try to test it first on your decals so there are no surprises .