Fuel Tank Hatch repair

[RWilson2526]

got my fuel tank hatch off , old wood removed and all sanded/grinded and ready to epoxy the new wood down...I have some filler to thicken the epoxy resin as was suggested on some of the posts here to make sure any voids are filled. What kind of viscosity am I looking for when adding the filler? still fluid enough to roll on or more of a pasty trowel on type of consistency? this is my first ever work with resins so its all new to me. I havent read the mfg literature yet to see if there is a max allowable before comprimising the strength of the stuff but thought I would ask here.