I currently only have one 3/4” layer of MDF on my machine for a spoil board. What is the reason for two layers besides more room to re face it?
Well if you add t track or make t slots you’ll need more layers
True, but I don’t plan to do that. I use the Spiker clamps.
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A single 3/4 piece is fine.
As to resurfacing your spoilboard, just use the surface of the spoilboard as your zero in your CAM (all cuts are in the positive) and not the top of your stock. Stock can vary, your spoilboard will charge only with exposure to moisture.
It is always better to not spoil the spoilboard. And it’s not that hard to do too!
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Thanks! I was just trying to figure out if you needed the extra thickness for rigidity or dampening effect. I did make sure to align the supports and make sure they were on the same plane.
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Cool, sounds like you are off to a great start then. The spoilboard does reduce ringing but making it thicker doesn’t add any benefit.
I use a thick spoil, 2 layers sometimes 3 of .75 mdf so when I shoot t25 screws to anchor, I ain’t got time for rails and fancy holes in it… they don’t call it spoilboard for nothing…
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I used 3/4” HDPE for the spoilboard. The MDF doesn’t provide a stable surface where I live in Charleston, SC due to high humidity.
It is thermally and moisture wise stable. Added benefit is it is very heavy and eliminates the need for drilling holes for hold downs. Shim the Y axis on one side with shims cut from the scrap, attach 1/8 aluminum plate to the frame on the 2 X axis frames. Other benefits, eliminates the need to cut and insert tracks. I got a router bit from a company named tool quest that cuts the tracks in the hdpe. Slide hold downs for the workpiece into the tracks.
The HDPE spollboard adds rigidity to the frame.
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I used to have a spoil board, 1 or 2 18mm mdf, but found they reduced my z clearance when doing 4th axis or 3d work so now i don’t have any spoil board. I use slots in the frame as T tracks and scrap timer under work peices when needed, seems to work well and i can clamp deeper work peices of work to the side of the frame rails when required.
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I like it! Gives max Z for sure
I just use one layer of MDF - seems solid and flat. Yes I do always zero from the board but quite frankly I still often cut slightly into the board to ensure I have a complete cut. I hate holes etc in my board. I just screw down my pieces onto the board - they seldom lift (unless I really mess up of course - which does happen) and never move. I dont have to worry about hold downs or T tracks - or cutting T tracks - which for sure I would do. I do mess up - as I suspect most mortals do.
You can simply resurface a few times and then take off the board and put on a new one. Simple easy and you retain most of your Z travel. I cannot imagine doing all the work I see people do to create a board that one way or another you will destroy if you do a variety of things with your machine.
I completely agree with that assessment !
Yikes 
I gotta admit I have never had that problem. Clean cut-outs every time. Faint little traces all over the spoilboard.
I have a PCB mill with an aluminum spoilboard. I surface it about every 2 years whether it needs it or not. It touches off to the spoilboard by default. That machine stays running from 8:30 AM till about 6PM so its being used. Its just a perfect 0.001" repeatability so I guess that aint fair 
For the record, none of my heavy mills touch off to the table. None of them can even reach their tables 
Which is why they have monster vise on them. For metal its always Z negative in cuts however I do have flats on the vise to touch off on for better accuracy when using 3D printed jigs and fixtures.
I doubled up on mine to increase the rigidity. I was getting movement. When I tried to tram, it would move a few thou just basically touching the table. It is a lot better now. it takes force to move it. The other benefit is while I am learning to “not spoil it” I can resurface a few more times. But the amount of less movement is a huge benefit to me. Pocket clearing is way more consistent now. I recommend it, as a newbie myself.