I have been experimenting with washes to start learning weathering and keep getting the same problem.
The wash goes on and then dries to look like accumulated fine off white ash in all the nooks and crannies.
Model painted some years ago with Humbrol enamel and then sealed with Games Workshop Citadel Purity seal rattle can. Wash applied about 5 days later.
Wash is Humbrol enamel thinned with Humbrol thinner as found by searching here.
I've been told that the paint could have gone off and the pigment is being carried out of the paint base. Has anybody heard of this? Any help gratefully accepted!!
I suspect the flattening agent in your GW sealer has been lifted out by the wash and redeposited heavily in the nooks and crannies, hence the ash appearance.
I would recommend you do all your primary painting in acrylic paints if using an oil based (enamel) wash. If you do not desire to switch paints, at a minimum change your sealer or clear coat to something acrylic based like Vallejo Satin, or a lacquer based flat finish like Testors Dullcote.
From experience I can tell you do not be tempted to change to Humbrol's enamel flat finish. You will simply get the same if not worse results.