I will paint and decal the model.Basicly It will look like a new plane at that point.So now you want to wear the whole thing down to make it look like its been working outside.
Now thin down the white or grey realy good.This makes it translucent so it doesnt spray on realy heavy like another coat of paint.I will then spray a mist of that over the model.Do this lightly in steps until ya have ya have it to the state of wear you are looking for.Usualy I wear down the top more than the bottom.The reason being the top sits in the sun all the time.
Remember the decals reprisent painted on information that is going to wear just as much as the rest of the plane.So you build it as new and then wear it down from there.I tend to look at real world working items for hints on weathering.Such as Trucks,cars,fork lifts at work,trains.Have a look at something like that and see how it wears where oil leaks from where dirt builds up.
Tape was a good idea.I tried that as well but like you said it doesnt keep the paint out very well.The foil works much better.
Aslo a hint on decals.Paint your model then spray it with a gloss coat.Then apply your decals.The gloss coat keeps the transparent part of the decal from being seen.After the decal is applied you apply a flat or gloss coat over top of it to seal the decal.
Pipz
