It really depends on how aggressive weathering you wish to achieve. On rolling stock or airplanes i start with #2000 and go to #12000.
If you want to heavily weather decals on walls etc, i think you could start with #600 or #800. As always practice/experiment first on some scrap.
I don't know the answer. But: emory board is better than sandpaper. More flexible. OTOH I do use lady's finger polishers/not nail files. I bought them at a Dollar store and they are on sticks. Some are very rigid and harder to work with. Some are just slightly flexible and easier to work with. I know this may not be your choice, but I thought I'd give you MHO & FYI.
On my rolling stock, I usually use the scotch pad stuff, the finer material, like the scrubby pads you use on dishes. It does a good job of fading the decal, or the numbering on a freight car or loco.
I would think it would work on buildings, too. Anything sandpaper seems a little rough to me, but, maybe something like the 600 wet/dry stuff?
Mike.
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Can someone recommend what grit sandpaper I should use to age the decal once it's set and sealed?
It's been a while since I weathered a decal on brick and wood.
A search on the topic didn't discuss the sanding technique.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!