Looking for help for those more expereienced than I....

I've gotten fairly decent at splitting cards and have started experimenting in gluing them to make double facers.

My first was 6S (which had the two layers; face and middle core) and 3C that was a single layer. The 3C was pretty wrinkled and I was worried about how it would look.

I used Elmers no wrinkle rubber cement, putting layer on back of each card and letting it dry. I then lined them up and put them together. I used dowel to roll over them, and was pleasantly surprised the 3C smoothed out and looked good. I put the the double spacer in between some old cards, then sandwiched that between two peiced of wood and clamped it all down.

After letting it dry for 24 hours I took it out. The thickness of the card is spot on. And while the face of 6C looked great, the face of the 3C has a slight grey stain all over the face. Sort of a mottled grey background. I thought it just needed to dry more, but it's been 2 days now and still looks the same.

Any idea on cause and how to avoid this?

Perhaps I used too much rubber cement? Would that cause the blackish middle core on the 6S to show thru the thinner single layered 3C?
Should I have used 1200 grit sandpaper on the black core? But would that then make card too thin?
I've read about dry transfer paper...is that route to go? Does it make the card too thick?

Hoping to learn from others....

Is it actually a stain or is the paper of the 3C so thin that you're seeing the core of the card? Because that's what I was getting all the time.

I resplit the glued card. When I laid the single layer 3C face over a blue card box, I could see shades of blue through the card. So guessing it's the core showing through.

Did you solve the problem?

On and off, I'm not great at splitting either. Sometimes I split better than others, but if I don't get enough of the paper off along with the printed side I know I'll have the problem. I usually just throw it away at that point.
Newer cards work better, getting it done quickly rather than in little bits seems to work better, but then I end up tearing the face as often as not. One of these days I'll need to split a few decks worth and I'm hoping that at the end of it I'll have a consistent feel for how to do it right.