Skimmed waste board into washboard LOL, 4 questions

Today I measured the different heights on my wasteboard with UGS and a bit of melamine and found a difference of up to 1.2mm in different places. I made a skim file in easel and cleaned off 1.2mm on a 75 x 71 cm square. It’s now much better,

BUT I got the washboard effect where the bit ran in the x axis, although it looks pretty good in the y axis direction cuts .
I remember from another post (that I couldn’t find again just now) that this means the z axis needs a piece of aluminium can to shim the bottom out a fraction. However, in the meantime, Question 1 will the ridges have any effect on carving? I can see that the board looks much closer to level, i.e. the edge nearest my zero is now a bit over a mm into the mdf and on the far corner and back the bit barely scratched the surface. (This was where I was 1.2mm lower). Question 2, would rerunning the skim cut but only going up and down in the y axis on my free day of Easel pro make it smoother? Is there any point in that? Question 3. the mdf is now kind of hairy, would a coat of varnish (not water based) help keep it from sucking up humidity? Is there any point in this?
And question 4, if I do manage to tram it up a bit or re-run the cut, how fast could I run it as watching the skim cut while breathing in mdf dust was possibly the most boring hour and a bit I’ve spent this year. (I used an 18 mm flat bit, 1016mm/min, one pass of 1.2mm)
Thanks for any suggestions!

I would expect the tops of the washboards to be at the same plane, so the work should sit on them with no issues if it spans them. I guess it is possible that if the work is thin, and you are using some type of double sided tape to hold it down, it might bend some in the low spots.

1 - The ridges wont have a big impact on larger pieces.
2 - Yes it might, if your router in only leaning in th Y direction a Y-only skim cut with small stepover will provide a smoother surface. But it will only mask the symptoms so its better to spend a little time tramming your router up :slight_smile:
3 - You’d need to seal all faces of MDF for it to have an effect, I’d say just run with it and replace when necessary.
4 - Depends, go shallower like 0.5mm now and try feedrate 2x of what you did. Also set stepover to 80% (default is 40%) which will give a shorter total tool path needed to cover your surface.

Thank you, I’ll leave it as is for the moment, and look into the tramming posts again.