what do you mean by fillers? HDU dents and chips, pebbles from transporting
puts holes (dents) into it....I fill those with spackle paste, just have to be
careful, cause when you paint if the spackle isn't properly primed and smoothed over well, you will see the spackle (shows up shiny), ...
depending on size of spackle, I sometimes prime first, then spackle, sand, prime with BIN, then start putting on final coats, and of course, the more
you paint, the less you'll see of the spackle...
maybe Precision Board is more porous than Sign Foam, hence the fillers, but if I was doing it, I wouldn't use any fillers unless it was absolutey necessary