Vallejo Weathering FX - Slimy Grime Light

[Deus_Ex_Machina]

I just purchased the product in the title with the intention of creating toxic sludge effect on dark muddy bases. Is there anything to consider when applying this product? Maybe undercoating with white colour first on the muddy base? I am asking because a few months ago I applied a Vallejo crackle base effect and the bases for that project needed to be covered first with paper otherwise the crackle effect would prematurely dry out and flake off. 

[Firedrake Cordova]

Vallejo's tutorial implies just paint it over the (painted) surface you want to weather.