Should I Have Left the MDO Primer On?

I recently built a mailbox post sleeve, hollow to fit over a 4×4, to replace my existing one. The main body of the sleeve is MDO, chosen for durability outdoors. My design called for two laminations at the base for a more beefy look, with a single top-to-bottom layer in the core. The only MDO I was able to find was already pre-primed. I sanded off the primer in order to make the glued-up laminations for the lower section. My concern was the strength and durability of the joints on a primed surface. Was it necessary to sand off the primer, or was it overkill? – Phil Extance

Chris Marshall: Your application in this situation is more decorative than structural, so I suspect the pre-primed surfaces would have offered a good substrate for the glue joint. Really, the pieces only need to stick together — they aren’t carrying a load. With the primer now gone in those spots, make sure the seams between the glued pieces are well sealed with caulk and a good coat of primer/paint to keep water out and off of the “bare” MDO underneath.

Tim Inman: MDO is also commonly called “sign board.” It is usually pre-primed to make it ready for its intended purpose — signs. MDO is not a structural material. It is a smooth material, not necessarily a strong one. Since you were only using the MDO for visual impact in your project, it would be my view that gluing directly against the primed surface would have been completely adequate. To go just a step further, though: Your thinking is on target. Glue sticks to — and bonds with – -whatever it touches. So, the glue line cannot be stronger than the bond between it and the surface it is adhered to. If the primer has a stronger affinity to the wood than the glue, then gluing to the primed surface would produce a superior glue line. If the primer is weaker than the glue, then that would be the maximum strength of the glue-up, and you would get a superior bond by removing the primer. MDO is a poor choice where structural strength and integrity is needed, though, so this discussion is pretty much academic. MDO is smooth, pretty stable, and paints well. It looks nice.

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