Don't know if it's the resin in this set of Forge World walls I've got, but having a nightmare with the black spray staying on. I stripped and washed the models with warm soappy water as well as soaked in diluted Cilit Bang. Stuff is still flaking off.
Sprayed up some FoW Hano-Mags with the Desrt spray and when painting up noticed that there were hairline cracks all over the place. Will check my Panzer IV's & Panthers later, if they are the same I may change product. Would be a shame as I feel their sprays cover the model quicker than GW's.