Why is trim made of finger-jointed wood more vulnerable outdoors than clear lumber?

Finger-jointed trim is made by gluing short lengths of lumber end to end, which uses material efficiently and produces straight stable stock, but it introduces a joint every foot or so along the piece. Those joints are the weak point outdoors. The coating over them is what keeps water out, and coatings age, crack and lose adhesion — particularly at a joint line, where the two pieces move slightly differently with moisture and temperature. Once water reaches the glue line it works into it, the joint opens, and water then travels into the end grain on both sides, which absorbs far faster than the face. The result is characteristic: swelling, opening and decay at regular intervals along an otherwise sound-looking piece. The finding is a maintenance one — the trim depends absolutely on an intact coating, including at the ends and the back if it was back-primed.

Finger-jointed trim has glued joints at intervals along its length, and as the coating ages those joints open and admit water into the exposed end grain on both sides.

Finger-jointed trim is assembled from short lengths glued end to end. Its performance outdoors depends on a maintained coating, since coating failure at the glue lines admits water into end grain and causes joint opening and decay.

A Joint Every Foot, and every one of them is waiting for the paint to crack.

Check ends and cut edges especially. Trim cut on site and installed without sealing the cut exposes end grain directly.

• •Not recognising the regular interval pattern as finger-jointed stock
• •Overlooking unsealed field-cut ends
• •Reporting the coating failure without the resulting decay