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Is it ok if I get a craft knife , scratch of the line, and paint over it or do I need to reapply primer on the line?

you don't have to use spray primer but you would want to paint it again with whatever your primer color was then the base color. example say you primed black and then dark angels green. if you paint dark angels green over the bare plastic it will most likely be noticeably lighter than over black.

You dont need a primer, just paint on plastic will be fine for a small area like that.

You may struggle is it's metal and a very clean carve. Otherwise you should be fine. Make sure to just use a dab of paint the same colour as your primer, so that any base colour you add will match in.

You don't "need" to do anything, but there's probably a few things to keep in mind in deciding if you need to touch up an area...

1. Paint won't stick as well to the unprimed area. That may or may not be a problem depending on the model.

2. If you're painting over the area with a paint with poor coverage, the unprimed area may stand out. eg. if the model is grey plastic and you prime it white, then paint red over the top, the area with no primer is going to stick out like a sore thumb. If you paint the model a deep blue (which has good coverage) you probably won't notice the unprimed area.

3. Whether or not there's a ridge created by scratching away the primer. Obviously the primer has some thickness to it, when you scratch it away sometimes the paint will tear and you may be left with a ridge that may or may not be visible later on. Depends what primer it was, how thick it was and how you scratched it.

Even the most fastidious of us may miss removing a mold line in the building phase that is not discovered until after the paint and shading.

I agree with @AllSeeingSkink - depends on the material and color. Try a spot without primer and see if any color issue. Usually, it is not critical unless it is a huge model and a very noticeable area.