We have found that the lawnmower effect will disappear obviously with uni-directional but you need to also make sure lamps are at the right setting. We have played around with ours a ton and found that if you only use one lamp, like user side, it will produce a nice print and if you just use the service side lamp, it will produce a "glossy" print.
Take a test print decent size on foamcore and run the same print bi-directional with each lamp and you will see an amazing difference.
We have found that a regular lam will definitely hide this effect, which shows up more prominently with darker colors. The liquid lam is great, you just have to watch it on certain materials where the adhesion isn't the greatest. But what we have really found out is that we spray all our liquid lams on because the rolling puts down too much lam in our opinion which makes the inks "soft". The spraying is really easy to do and you can control the look and dries a lot faster.
If the calibration is setup correctly on teh 9840 for each material, your lawnmower effect will go down significantly so talk to your tech about that. It's the same principle with roll machines is that you have to do a media feed/media comp adjustment for every single media you put it the machine because of thickness, weight, etc so why wouldn't you have to do one with the flatbed? Unless you have a stationary bed, then it would only make sense that different materials are going to require different media rates to ensure the best look, etc.
Talk to you soon and feel free to email or call anytime,
Best,
Christian
chris@thecsco.com
513-241-2726