Making Custom "Wash Me" Decals

There are several variations of fonts called 'Fingerpaint' that would probably do for the effect you're looking for. As to size, it's simpler to resize the text/image than to dial in a precise font size.

Assuming the text will be the clean white or clear window,

it might be easier to write the words on the model with white glue, weather over it and then pick it off.

just a thought.

That's a good idea GM.  Like maksking off windshield wiper marks on a dirty windshield. 

This may be harder than it sounds.  You'd have to use the clear decal paper, and make the dust effect part of the decal.  The lettering would have to be bright white.  The trouble would be making the dust for all the truck surfaces.  You could do just the surface where the writing would be with decal, but the challenge there would be matching the dust on the decal sheet to the dust on the rest of the truck.

You could do a piece of scrap with your weathering paint and scan that to match the dust color- the challenge is matching the transparency/opacity.

Thanks for the info so far, everybody.

@Don:  Couldn't I make the lettering tha same color as the dirt/grime, put those down first, then apply something like a streaking grime pigment (Vallejo or AK Interactive products)?

I have done this a few time's .

After some "trial and error" I hit upon a solution that work's. using a small tiped paintbrush, I dipped it in a jar of Decanted Testor's Dull-cote and hand painted "Wash me" on the rear window of a muddy truck. It looked just like someone had taken their finger's and written it in the mud.

I have also done this to several 1/87 scale Southern Pacific diesel locomotives

I weathered them first, then painted the "Wash me" on the side's ( If you know the Southern Pacific, their loco's tend to get pretty Grimy. )

There's a bit of "Trial and error" to getting just the right  touch to it . Dont load the paintbrush up, just a bit should work. And if it doesn't, you can alway's go back and do it again.

the letter's should look a bit rough, remember, people do this with their hand's so "Imperfect" letter's are what you want.