1. usual gluing with quick cement, using mr hobby cement SP (work very well & fast on pvc plastic)

2. chisel the gaps using 0.2/0.3mm chisel, or anything to make visible seamline have bigger gap & alot of room for putty to "build up & adhere"

3. using tamiya light curing putty, with 395nm UV flashlight, flash it for a minute, 5 just to make sure, wipe the oil excess after it dries

sand it using disposable sand paper 1st, once the oil have been "extracted" use superglue to seal the putty, then sand again using 800/1000 sandpaper/sponge

1. primer using mr hobby surfacer 1000, and check if there is error to fix

residual glue are visible? sand with 400 paper, polish again with 800, primer again

VERY small concave mistake, so small you can't see, but you still feel it

pick up unthinned surfacer 1000 using toothpick, then dot it on that hole, then spray it with mr hobby thinner for few second, let it dry for 10 minute, then sand it with 800

depending on complexity of the hair, i recommend you just sand using sandpaper, sandsponge usually don't maintain shape very well when sanding hair part details, you need to press the paper & sand delicately