Yes, untreated, the inside of the cowl will absorb fluids. The minimum i would recomend is filling the surface with a coating of resin. The usual drill is to use up the leftover resin from other fiberglass work since you can do it a section at a time. I went the whole way with mine, filled the surface with resin then sanded, primed, and painted the inside of the cowl with the same Jet Glo that I used on the outside of the airplane. The result is a cowl that is super easy to keep clean and looks sharp 6 years later.
John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA