Next week we try to infuse a canoe in flax fabric, a big challenge for me because it's my biggest project for so far. I made a few test panels to figure out the lay-up composition, but it was not completely perfect...
I had a few places where the surface was bad, but I have no idea what's the reason. I've worked with MTI hose and clamped off the resin tube when the epoxy reached the MTI. I infused at 13 mbar, so is it possible that the vacuum was to much? I did the infusion very slow, so I don't know what I best can change during the next test? The second picture is the backside where I've put some Corecel ribs, but here I think it was bridging from the peelply.
Is it common that they do a post-treatment after an infusion (sanding and varnishing)?

I'd say the majority of your problem is the bridging. When you've got major bridging like that, it will cause issues not only in that specific area, but also in the rest of the laminate as it causes uneven resin distribution within the laminate.

Be 100% sure you have no bridging on your next test.