I live in Haiku Maui. MDF and Haiku don’t get along well long term due to humidity.
Any suggestions for cost effective material to replace the MDF spoil boards that won’t absorb moisture like a thirsty sponge?
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The foamed PVC sold as plastic lumber seems a good option — I used it on my SO3:
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Not sure about cost effective but there have been those who have used HDPE. It comes in varying thicknesses, up to 0.75 inches. Will not absorb moisture.
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It just makes you cry when you cut into it
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I would totally use the PVC boards as @WillAdams suggested. But in my area none of the big box stores carry them. I also live in Salt Lake City, UT… so a desert and opposite of HI, so likely why I can’t find it here.
Do stores near you not offer a ship and pick up at store option?
https://www.homedepot.com/p/Veranda-3-4-in-x-2-1-2-in-x-8-ft-Cellular-PVC-Trim-H190JWS5/100243235
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I’ve been looking into the same thing, a material that is tough enough, waterproof, and cost effective.
I initially thought about PVC, but it seems a bit soft. The PVC I use for signs & decorations is the foam core. I haven’t seen any solid core locally.
Then I considered something like the Trex decking material, but it’s hard to find solid pieces, and the widths for deck boards are 5 1/2", not enough to get two 3" slats.
I recently saw some lawn/beach chairs made from recycled HDPE. I’m going to see if I can source that locally.
I’d like to explore the option of milling out some channels in perhaps the 4 center slats & adding vacuum for workholding on jobs that it’s appropriate for.
I redid my deck a few years ago with this:
It’s a lot more expensive now but it wasn’t exactly cheap then. It is a mixture of HDPE and Fiberglass with zero organic material with Trex and some of the others have. It machines very well, I wish that I had my CNC when I did the deck for some of the trickier cuts.
It does have a lot of little pinholes when you get below the HDPE outer shell. For most cases that wouldn’t matter but may not work for your vacuum case @Tod1d.
Are you thinking independent channels on each of those 4 center slats with a separate vac line in each with valves?
EDIT: oh yeah. You can get it in 7.5” widths.
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Nope. They won’t even ship it here. 
That is really odd. I wonder what the restriction is.
I’m guessing the exterior stuff is just not sold enough here to even bother with the logistics? I can get Trex deck stuff, but it would be horribly expensive an inefficient. I can get the interior trim boards, but it would cost around $130. Might be worth it if I cut more often with coolant, or lived in a humid environment. I’ve thought about HDPE, but that would cost a fortune and most things I use coolant for, get run on the HDM with tooling plates and vises. 
I’d think in this case zeroing at bottom would be most beneficial.
Yes. Not foolproof though. Had a bit get pulled down last week and carved into a pristine spoilboard.
What if you were to seal the mdf slats with shellac or danish oil?
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Shellac would be ok, probably 2 or 3 coats.
I don’t think Danish Oil would be a good choice. It may actually cause the MDF to swell some and it would take a while to dry.
I seal my mdf with a couple of coats of clear lacquer, Soaks it up like a sponge, so I go for three coats, and then a light sanding.
This may seem off the wall but I bought an SMW fixture plate over 3 years ago. Best decision I have made regarding baseboard and rigid stability. It was expensive and I had it shipped to the UK but it has repaid the choice many times over.
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I was researching sealing an mdf workbench recently, and found some videos on youtube. I don’t know if the oil swells the mdf more than any other liquid, but that could be fixed with a surfacing pass once dry.
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Interesting. I like Danish Oil and use it a lot on furniture projects but then topcoat it with shellac. I wouldn’t have thought of using it for this application as a sealer.
Shellac dries fast. I think I would stick to that for sewing the MDF if I intended to do it. My garage has AC so not that much of an issue for me but I may consider PVC the next time I replace anyway. The only downside is I might need sunglasses to work on the machine.
I like shellac too, but it’s biggest downside is it’s so expensive.