@Rodrigo - Welcome to the community! I had a very similar problem when I attempted the first leveling passes on my wasteboard. What fixed it for me was taking very shallow cuts, like 0.002 with each pass. Yes, it took quite a while to get it done, but its flat way beyond my requirements. Edit
I also sealed the wasted board with spray-on poly after I discovered that humidity changes would expand the outside edges more rapidly then the center.
There is some contact, so I’ll be mindful of that.
I think I found the problem. In going back upstairs to the machine and giving the Z carriage a good wiggle, I found a tiny amount of play. I tightened up one of the “inner” eccentric nuts to get rid of the play and it feels rock solid now.
Tomorrow I’ll try giving it a new haircut with it all rigid.
Hmm, that’s good to know. It gets pretty humid here in the winter (Portugal), so I’ll keep an eye on that.
As I understand it HDF Valchromat should be readily available to you in Europe. It’s far superior to MDF in terms of stability and humidity resistance. It also mills amazingly well. Caveat, this is just my impression from reading online, donno how easily you can get it in Portugal.
So to bring it to conclusion, I had to give it another 1mm haircut in the end to get the weird ‘stripes’ completely removed, so this wasteboard will not have a very long life of resurfacing ahead of it before it becomes useless, but live and learn.