[Willie T]

The humorous part of fabricating a 'backer board' is that it is totally unnecessary.

Think about it. What purpose does that pointed triangle shape serve?

None. And that back part can even get in the way if the ceiling to wall line isn't perfectly clear and straight.

I use simple firring strips that I buy cheaply almost anywhere. They are ready to go up, just as they are... although sometimes I do cut them in half to get smaller pieces.

 

[Willie T]

Bama,
I have used backers in the past. It really helps on the walls that run parallel to the ceiling joists where you don't normally have a nailer other than the plate on top. The important thing is to take a piece of your crown and fit it to the inside corner of a framing square. Make sure both flats of the crown are hitting the framing square and see how much space you have behind the crown. Make your backer a little smaller to allow room for adjustment when installing the crown. You also have to stop the backer short of the corner like it shows in one of the pics in the PM article. If you don't if will interfere with your crown end that butts into the wall.
Mike Hawkins

Those two are SUPER important. Never make a backer that fits tight up against the back of the crown piece. About a quarter of an inch space between the backer board and the crown molding is good.

This is adjusted by simply ripping the backer board if necessary. It usually isn't necessary.

 

[Willie T]

Undoubtedly the correctly angled cut is important. But there is something else equally critical. And that is making sure you get each piece of molding up on the wall in the same orientation as all the others in the room.

This is especially more critical as the molding size increases. It is really easy to get molding pieces up on a wall, totally mismatched to the angle of another piece coming around the corner... a piece that HAS to meet it correctly.

The pictures below show a homemade tool that makes that task easier.

I wish I could give proper credit to the person who first posted pictures of this 'tool', but I just plain forget who it was. Still, I made a tool like they showed, and use it all the time.

 

[Willie T]

I have two of these black ones, (lower left) and seven or eight of the orange ones (middle bottom)... and two I made from wire. (the big pictures)

And over a dozen of the spring clamps. (lower right)