Redwood for flatback braces

Sitka spruce and American redwood are about the same density, around 28 lbs/ft^3. Western red cedar is even lighter, around 23 lbs/ft^3.

Of course, those are average figures, individual samples can be higher or lower.

 

I would stick with spruce. The redwood or cedar can be strong enough if sized appropriately, but spruce is 'springier' and keeps its shape better. This is important in your application. Guitars built with redwood or cedar tops have spruce bracing for this reason.

 

You could give it a try; the worst case scenario is you'll pull it apart and make them all over a few years down the road.

I used a LOT of redwood during the couple of decades I lived in the Pacific northwest- 'probably built 50 acoustic guitars out of it, and threw hundreds more in the fireplace that were rejected. The textbook measurements for sitka and redwood may be similar in the reference books, but redwood tends to be all over the map in real applications- some is incredibly stiff and lightweight while other is a mushy mess. If your wood is from the Lucky Strike tree that the Carters cut by hand in Northern California several decades ago, that is so stiff and nice that it is in a completely different category than any other redwood I've ever seen or used; 'almost like a rust colored red or Adirondack spruce; legendary among west coast guitar builders.

Red cedar is similar- several folks have used it successfully, but it has a lot of variability.

Arnold, 'you out there lurking tonight? How is that big redwood new bass on your workbench coming along????

 

I'm getting ready to set the neck in the next week. The Redwood is incredibly stiff for its weight, with a tap tone that includes a distinct upper register. Because the wood is soft and prone to damage, I installed an oval of maple veneer in the soundpost area, and I carved the entire top about 15% thicker than spruce. I also made the edges a full mm thicker than usual.

As to Doug's question, I would think Redwood bracing would work nicely, but the area under the soundpost will get badly dug up unless you reinforce it there.