What causes build lines in Z direction?

I’ve been having this issue since I got the machine but there is no consistency as to when it happens or where the part is when it happens. I get these “grain” lines in the Z direction, not your typical X and Y like you would see on a traditional SLA. I’m using version 5 tough resin on thinnest layer possible.

I would check the optics for dust, might be some on the glass, bottom of the tank and possibly on the main mirror.

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So the piece of glass that’s directly under the tank came with a smear on it…kinda looks like when you spray windex on a window on a hot day and wipe it it and it leaves that streak. But that’s only on 1/4 of that glass in the corner.
If it were on the mirror, wouldn’t it be more consistent?

There might be tiny specs of dust you may not see directly on but if illuminated at an angle they would show… I would suggest opening a ticket with FL and they can direct you on how to clean or remedy it issue.

That might be due to your model, for curved surfaces you need to have a high enough subdivision level so that you don’t see the polygon facets. So for example with a cylinder shape, you might change it from being 24-sided to being 48-sided.