I always use vacuum infusion technique to manufacture my CFP, but weight matters with me, and I want to limit resin uptake of balsa wood core material, so I was asking if I put the resin myself on the CF layer up and down the wood core material then put it in vacuum bagging, can this help me on decreasing the resin taken by balsa wood?
If not, please tell me how to decrease the balsa wood resin uptake.

Yes I want to use grain balsa with thickness 6.5 mm, We manufacture car monocoque chassis, we are using foam last years but in some areas we need balsa properties.

Can you tell us how to seap it using epoxy ?

Balsa is always a bit of a risky material as it soaks resin so easily.

If a wet layup and vac bag is done then you run the risk of the balsa pulling resin out of the laminate and that makes a weak bond between the core and the laminate.

If infusing then it would infuse/impregnate the balsa right through, could be quite ok and super strong but really heavy.

If you are only using small patches of balsa maybe you can pre seal it with a resin coat and let cure, then when you vacuum bag a laminate to it it should not draw any more resin in or take any out of the laminate.

Personally I would stick with foam, there are plenty of higher density grades to use and less risk.

I agree with what Fasta said about the trade-offs.  Higher density foam is easiest but balsa is hard to beat for compression, shear or price!  There are manufacturers of Balsa that make pre-sealed infusion balsa that soaks up half (or so) as much resin as standard balsa.  Gurit Balsaflex, Corelite PC11 are examples.