Color rubbed off on PU synthetic leather

I have an HP 375, we bought it to print on PU synthetic leather. Colors print nicely, but when I apply glue on the back and make it into a notebook cover, the color rubs off easily. Even when I don't apply glue on the back of PU, printed color still rubs off easily. Has anyone else also print on sythetic leather before and had the same problem.

#2 we have tried using Alpha3 TD603, since this is the only recommended choice for PU leather. Then we've also tried several others because we prefered result using 16P and with Alpha3 TD603 highest pass allowed is only 12P. At the end we modified HP light textile display banner to 16P 150icc. Person who sold us this machine recommended us increase temperature and increase to 20-24P. As well as increase percentage on OP. So far we have increase temperature, but we have not found where to increase OP. We've also printed on paper and Canvas, but PU leather is the only one that's giving us problems.

#4 Hi, to edit the optimizer level, just select edit the profile in the substrate library, there is a advance tap at lower left corner, then you will find the optimizer level there.

We've tried using 35% optimizer. After it's done printing, we used an eraser to check if colors fade again and as you can see on the picture that after using eraser on the PU color fades. I know it' s not normal to use eraser on PU, but it's really important that color stays on when with friction.

Thank you Sonia, We've decided to go with your recommendation, we'll be applying varnish on top to prevent any discoloration on PU. It's quite a relive to finally found a solution. Thanks again.

#8 What varnish did you use?