I’m building a sofa with 30 mm ash rails wrapped around a 20×20 mm steel tube spine so the silhouette stays light but the span doesn’t sag; the dry fit took a 220 lb center load with about 2 mm deflection over 1.8 m. Before I commit to powder coat and glue, how are you handling wood–steel movement at those tight mortise interfaces to keep joints silent long-term?
I keep the steel-to-ash interface quiet by leaving about 0.3 mm clearance and wrapping the tube with 3M UHMW tape 5421, then bedding it in a thin bead of Sikaflex-221 so the joint has a controlled “slip layer” without play. If you powder coat, mask those contact zones and wipe with xylene before tape so adhesion doesn’t creep — what span of seasonal swing do you see where you’re?
I’ve had good luck sliding a 1 mm adhesive-lined heat-shrink sleeve onto the “20×20 mm steel tube spine” only where it bears in the mortises; it adds a tough, slightly compliant skin that keeps the joint quiet long term. Allow a hair for the powder coat too (about 0.06–0.08 mm per face) so you’re not pinching the shrink; will your 30 mm rail mortise take that extra thickness?