soundproofing will sag for def whilst water logged but unless in really bad condition in the first place it should be ok,dont hold the lance too close to it and wires etc(as stated above)
get your engine nice and hot before you do this,makes it alot easier for the solution to break the crap down on the engine,make sure you make enough solution to keep re-applying to the bay to stop it drying out,leave for 5mins or so and wash off,
close bonnet and get engine hot again to dry the bay out, now hot and dry you can apply your chosen dressing,i use highstyle from autosmart.