For me, depends on what the customer wants..
For my own games, I strip decals, sand (hand & belt) - as MarkC suggested.
Once back to bare wood, I rub some prepsol on it to clean - but also helps show small dings etc. Clean with dry rag.
Fill spots with builders bog, once set hand sand (with float) til silky smooth!
Next, prime (2k) or (acrylic) with primer / filler. Depending on the mix is how much fill it gives. 2:1 is OK in 2K and is really good for filling some minor graining & pin holes.
Once primed, I lightly rub back and make sure is nice and flat. I then apply paint to the whole cab. Not everyone does this, but I find it helps ensure a nice even flat surface, as decals will show every bump, nick and bit of dust.
Once painted, I rub back again (taking off shine), tack rag then decal install (wet or dry).
