Many years ago MR had an article on putting down decals with diluted white glue.  I hadn’t tried it at the time but at one of our Missouri Pacific Historical Society annual meets Lee Freeman brought a scratch built O gauge T&P Muley caboose where he’d used the white glue technique.  The decals looked perfect.  

Here’s a CB&Q car I just finished and added Smokebox decals. I used their BCR decal first and then overlaid the weight and location using white glue.  The glue dries flat but I’m open to other suggestions; I’ve got a bottle of Future handy  

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Charlie Duckworth 
Omaha, Ne.