Tough decision. I'd think the MDF would make a cleaner insertion hole but that the ply would be more durable. Assuming that the spikes will be in the base panel which will be holding the whole unit, I think I'd prefer a quality plywood panel.
I found some supporting opinions for plywood here; Proper Screws for MDF - by John R. @ LumberJocks.com ~ woodworking community.
My plywood is not of marine quality though.
My plywood is not of marine quality though.
Probably depends as much on the inserts as the core materials - thread spacing/ pitch angle depth can vary quite a bit on these.
While it may seem to go without saying, care must also be taken to pilot hole the correct size - too large and the thread won't bit deeply enough to guarantee against the wobblies - too tight and they will be an absolute bitch in screw in all the way, and particularly the Zamac etc softer cast inserts that are installed with hex key can snap before you're done. Good luck getting those out.
That said, good quality multi-ply has better thread holding properties than MDF.
While it may seem to go without saying, care must also be taken to pilot hole the correct size - too large and the thread won't bit deeply enough to guarantee against the wobblies - too tight and they will be an absolute bitch in screw in all the way, and particularly the Zamac etc softer cast inserts that are installed with hex key can snap before you're done. Good luck getting those out.
That said, good quality multi-ply has better thread holding properties than MDF.
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