home depot carries a product made by "Goop" it is a 2 part epoxy putty which should chemically bond to your synthetic stock material. sands and paints very well. Also less tendency for small air pockets than bondo.... by a long shot. Custom synthetic stocks are a side business of mine....and the goop marine fix fast has become my favorite epoxy filler. They also make a high heat version which handles temps exceeding 250 celcius and minus 30. which i use as a bedding compound in my semi auto stocks. The marine stuff is dirt cheap..... under 5.00 i think for way more than you'll need to fill and contour those cross hatchings, just hit the area with a dremmel wire wheel to remove the paint, keeping with the grain of the hatch marks, then clean the dust away with a tooth brush and mineral spirits or hot water, wipe it dry and yer good to go.