Hi.  It seems print-through can be caused by loads of factors scrolling through the forum! Any thoughts on mine....? 

I've been making some small flat parts (400mmx900m) as a trial/practice before making a larger (3.5mx1m) piece, using Easy IP2 polyester infusion resin, glass chopped strand and woven roving, and Easycell 75g PVC infusion foam (grooved and drilled)
My first lay-up was gelcoat ( 2 coats of dark grey at 300g/m per coat), 300g csm, 300g wr, 600g wr, foam, 600g wr, 300g wr, 300g csm, bagged to full vacuum, left to de-bulk then infused.  Ended up with print-through of the infusion grooves of the foam, which I would class as severe - you can feel them.
I did a second version, extending the cure time between gel coat layers and added an extra coat and left overnight.  I also added a layer of mat of 600g csm each side of the foam.  This part has much reduced print-through but it is still there, especially visible with the dark shiny gelcoat and would not be acceptable for my job.  Structurally both parts are sound, they're just cosmetically pants!
So:
Do I have to hand lay-up a layer of surface tissue over the gelcoat and let that cure before anything else? (is this normal for infusion?)
Is using the grooved foam the wrong product for just four layers of mat? (are plain foams used for infusion?)
Am I using too much vacuum, or rather, would a regulated reduced vacuum still work? (more kit to buy!)

It seems that a lot of things to do with composites is trial and error but it would be good to make something decent at least once..!

You should definitely NOT get the vacuum regulator. Print though has nothing to to with the vacuum process itself, but with the shrinking of the resin.
And that is the problem: polyester resin has a very high shrinkage while curing. The resin inside the grooves shrinks, pulling everything above inwards, while the foam around the grooves doesn't shrink at all.
There are many solutions
A: epoxy resin has a much lower shrinkage compared to polyester. But epoxy is also  mich more expensive, and you would have to buy a different gel coat.
B: the thicker the skins around the core, the lesser the print though of the core. But as you are infusing in one step, the print through would never fully disappear.
C: as you said, infusing the main part on an already cured surface layer (or two) would help a lot.

Thanks. Kind of what I thought.
I was hoping to avoid using my consolidating roller ever again but will have to dust it off to do a surface layer me thinks.

You could absolutely infuse the surface layer. Ideally with peel ply on top.

Yep, now after a couple more test pieces I'm going to infuse in two hits. All the layers below the foam first then the foam with the remaining layers.
It's been a learning curve but I'm going to assume this basic strategy for any other infusion projects with these materials.