Scratch repair

It doesn't matter. The whole thing has to be refinished.

You might want to remove the bumper in order to work on it, by the way. That way you can bring it into a shop to work on it, and paint it in a small spray booth when the time comes. Perhaps you can find a local woodworking shop which rents time, and has a spraybooth you can use.

You must refinish the entire bumper! There is no way to repair a modern automotive finish, particularly a base-coat/clear-coat one, such that the repair won't be obvious.

Such fine paper should not remove the topcoat. It only levels the scratches from finer paper, like 1500.

I'd say that 2000 is a waste of effort, because the effects of paper as coarse as 1200 can be taken out with a rubbing compound, leaving behind a mirror smooth surface.

If there are surface imperfections in the topcoat, such as a bit of spray stipple, you can remove that by wet sanding with something as coarse as 600. Or even more coarse, if you are gentle. Then go through the grades: 800, 1200. After that, rubbing compound.

In my experience, if you are hand-rubbing, and the finish is soft enough, you can polish after 800 grit paper. It just takes more elbow grease.

Even if the color matches, the finish will never blend in because of the different layers of transparency. Each layer that is applied must cover the entire panel being finished.

If you just apply the layers to a repaired area, it will show, because the new base coat, at the edges, goes over top of old clear coat. You aren't matching base to base, clear to clear, etc. That is impossible to do.

The color match isn't even as important if you do the whole part. So what if the bumper is darker by some fraction of a shade? That will be hard to tell because there is a visual separation between the bumper and the other panels. The eye can only distinguish small differences in shade or hue when they are adjacent. If they are separated by strip of darker or lighter color, the eye can no longer compare them, and will think that they are exactly the same. Try it: take a strip of one of those paint sample "chips" they have in paint stores and place a pencil along the boundary between two squares. You will now find it hard to see the difference between the two shades. Remove the pencil, and it's obvious.

Even if you do get an exact color match, at the point where the bumper meets the other panels, there is an angle. The light is never exactly the same on both parts, so in reality, they will never show the same shade under the same light.

Imagine if you were refinishing the six sides of a cube. If the sides get a very slightly different shade or hue of paint, it will be impossible to tell, since light falls at a different angle on the six sides.

Not doing the entire bumper, that's all.

Sand down the entire bumper, then apply the finish in the proper layers to the entire thing. Let it harden, then cut and polish. You probably don't have to go finer than around 1500 paper.

If you don't care about a perfect repair job, just take it as it is now, and polish out the dull area that was sanded with the 2000.

Like you said, the bumper will probably get scratched up over the next few years anyway.

You never know, you might even get into a fender bender and the whole thing will be repaired and painted at a pro shop.

By the way, if you sand the paint with 400 and then apply the topcoat, do you think the 400 grit scratches in the base coat will magically go away? In fact, the top-coat will enhance their appearance. The same way that the grain of wood "jumps out" under a clear coat, those scratches will also!

Don't sand the base before clear-coating it. If you must, then you have to do it to completion, progressively through the finer papers. It's probably a bad idea to polish it, because the lubricants in the rubbing compound will likely interfere with the adhesion of the topcoat. But you can progress up to the fine grits at least.

The base coat surface under the clear coat must be quite perfect, because the clear coat won't hide any imperfections in it.

what did you apply with? If it's with a brush... You cannot brush paint a car. Well... you can, but it's not gonna be pretty, and it's definitely not gonna match the factory finish.

Ray