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I have difficulty getting mitered moldings to have a flush surface.

Typically they get nailed into the underlying structure of the house, so any uneveness, which there inevitably is, translates to the molding.

Making a picture frame sort of thing with a spline would be difficult because the moldings are very long, like 10 feet long, so it is difficult to spline one to another.

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  • Seems like today's traditional fix to the problem you describe is to use caulk to fill in those gaps, also seal along the top/bottom run where molding joins the wall. A regular carpenters caulk is made for this very purpose. – K7PEH Jan 2 '20 at 3:26
  • @K7PEH Its not really gaps I am concerned about. It is when the moldings are not flush. In other words, their surfaces are not coplanar. – Treow Wyrhta Jan 2 '20 at 4:39
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    Are these flat miters (like door casing), inside corners, or outside corners? Is the molding flat against the wall (base molding) or "sprung" crown? – SaSSafraS1232 Jan 2 '20 at 18:39
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Casing, moulding, and trim can hide many sins, but as you have found out it cannot easily hide large problems with the underlying rough carpentry. This is especially so with problems related to nailing edges.

Installing trim is a bit of an art, especially since much of it is installed by someone other than the person who built the framing, or hung the door or window.

  • Make sure there are decent matching nailing edges, especially at the corners. A level or straightedge can be used to check for this, and then any necessary shims are installed prior, or in combination with, the trim.
  • Good trim requires a good amount of dry-fitting. Most trim can be tested for dip and sag with modest pressure (unless you live in an older home where sometimes it feels like they installed load-bearing trim). Either shim or provide a more stable nailing surface if things aren't lining up.

Trim is always an approximation of "true". It is an imperfect world, and trim is there to hide imperfections and provide easier to repair wearing surfaces. Getting mitres to meet nicely on 3 axes is probably why many modest houses have butt-joined trim!

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It sounds like you're talking about flat molding mitered picture-frame style, like door casing.

One thing you can do for these is to run a bead of thick CA glue along the end-grain to hold the two pieces in place relative to each other. Spraying accelerator on the other piece will make them set up almost instantly but then you won't have time to readjust.

Be ready to nail them up right after you do this, as the glue won't be strong enough to hold even a short run in place. It is just for alignment.

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