Sometimes either through my own mistake or parts shaking loose on their own, I end up with a large gouge along the edge of a part that looks like a mouse bite. This problem is more acute in older kits that do not have a rigid sprue frame (think old Airfix, Frog, short run kits ). If the resulting “bite” is a semi-circle about 2-3mm in diameter, there is a way to repair the damage. First, clean the gouge area and use either a small round or convex file to remove any jagged edges.
Select a sprue (you can cut them from the sprue tree) that is closest in size and fit to the diameter of the gouged circle. If not, you can also stretch a sprue, and use the ends with the taper to get the matching diameter. Bond the sprue to the gouged area with a strong liquid glue for polystyrene like Testors or Tamiya. Some plastic buildup around the joint is alright because you can always file it down later.
If the sprue is too thick, you can also pre-cut it down into a semi-circle sector before bonding onto the gouge area. Leave it to cure for at least 3-6 hours.
Once the sprue or filler section is secure and rigid, you can cut it down in size. I use a flush cutter to nip away at the filler until it approximates the size of the gouge. It is better to cut away small sections than to overdo it and cut too deep.
Using a razor, carefully slice the remaining filler until it is almost flush with the original surface.
Apply some fine white plastic putty to fill any small gaps that may be found and also as a surfacing agent for the filled area. Let it cure for 12-24 hours.
File down the filled area with fine sandpaper from Grade 500 to 1000.
You can see how the gouges are totally gone after the treatment and painting.