Fairing a junction

[Alex Yeilding]

I have a built-in bookcase with a 1/2" ply back that [nearly] meets up with a wall that is slanted in at the top at 45 degrees. The back of the case and the wall are painted with the same flat wall paint. I want to fair the junction of the plywood and drywall, probably with a 2-3" radius cove:

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Looking for ideas on how to do this. I want smooth transitions, to soften or even camouflage this junction as much as possible. Possibilities I have considered:

1) Joint compound or plaster, shaping the cove with either a piece of PVC pipe or cutting a plastic drywall knife to the desired profile. If I go this route, I can get good transitions, but will it crack? Shoudl I use drywall mud or plaster, given that it might be up to 3/8" thick?

2) Cut a cove in wood, then undercut the edges to make it the edge of the cove meet the ply and drywall. Glue to ply and use drywall mud or spackle to smooth the transition. Paintable caulk at the cove/wall transition.

Other ideas?

[Jamie Buxton]

In your drawing, it seems that the plywood is just somewhere in the neighborhood of the wall. If you attempt to use plaster to bridge the gap, it will surely crack and fail. However, if the plaster backs up to materials which are firmly fastened together, you should be able to get away with it. Craftsmen used to create elaborate interior decorations -- moldings and such --- with nothing but plaster.

[Alex Yeilding]

In your drawing, it seems that the plywood is just somewhere in the neighborhood of the wall.
More info: The back of the bookcase is 1 3/4" from the vertical secton of the wall. A 1-3/4" spacer board is screwed to two studs, and the back screwed to it, about 15" below the top.

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The back is 1/2" plywood, 35" wide, glued into a rabbet in the back of 12" wide sides of the bookcase. Would this count as two pieces being rigidly attached? I worry that it would not. I could make it a little more rigid by wedging some shims between the top of the ply and the drywall.

Another idea: What if I put some waxed paper on the plywood and built up my cove with plaster, so that it is attached to the drywall, but not to the plywood. Then use paintable caulk to "join" the two?

Someone else suggested bondo. I've never worked with it and am not inclined to learn on this project, but if it's the right way, I can make a mockup to practice on.

[Jamie Buxton]

Plaster is completely inflexible. If you can flex the plywood, it probably needs to be fastened better to the wall.

I don't think Bondo would work well for this application. The usual process with Bondo involves smearing it on metal, letting it cure, and then grinding it flush and smooth. On metal, which is harder than bondo, the sander naturally makes a smooth interface between the metal and the bondo. On drywall, the sander is going to gouge the drywall at the edge of the bondo.

[Ryan Myers]

I think you need to get some stiffeners between the plywood and the wall. Either blocks or shims. I would stay away from regular joint compound for the initial transitional coats, because it will crack

I would use DuraBond to transition the wood to the drywall/plaster, it is joint compound only much harder. This stuff sets up like concrete and is very hard to sand, so you want to get it as smooth as possible. It will bridge and fill large gaps and some flex doesn't bother it much. You will still need to use some joint tape to make the surface monolithic. You can use joint compound for the final finish coat.

[Ben Grunow]

I think this is a place for a wood molding as anything troweled on wil crack over time (or it might but who can hang their hat on might not).

Think about a 1x4 with a profile on the bottom and a small crown molding with the back beveled to match the slope of the ceiling and nail/caulk/paint it in.

Just MHO, of course.

Ben