Because Van's Gun Blue soaks into the metal rather than coating it, the texture of the metal remains identical before bluing and after. If you have a scratch, and you blue it ... you will have a blue scratch!
So to REPAIR a firearm, the texture of the damaged area must be identcal to the undamaged areas. (Shine reveals texture. Rough = dull - Smooth = shiny) If you have any damage to a factory blue, and you want it to be perfect again... you must repair the texture first. The question is; How do you match a texture without knowing what grit or level of polish was used to create it?
Here is an easy trick. Using an overhead light - tip the firearm back and forth in such a way as to make the reflected light streak from the light source pass through the damaged area. As an example, if you have a blood spot on a firearm and it has removed the bluing. Tip the metal back and forth so that the streak of reflected light goes from existing bluing, through the white or damaged area and back into the blued area. If the streak of light gets wider, fuzzier, or less distinct in the damaged area compared to the bluing you are trying to match ... it is too rough! If the damaged spot shows a sharper or clearer reflection ... it is too smooth! WHEN YOU GET THE REFLECTION TO MATCH, THE TEXTURE IS RIGHT.
The tools you use to create the correct texture are less important than your results. Whatever you make the metal look like BEFORE you blue it is exactly what it will look like AFTER you blue it! Whether you use steel wool, a buffing wheel, sand paper, a file, or drag it behind your car ... your results will be perfectly displayed.
I usually use fine wet-or-dry sandpaper. It is cheaper than Emery Cloth, comes in every conceivable grit from 80 to 2500, and is vastly quicker at getting where I want to go than steel wool. Most of my work is done with 320 as the roughest up to about 1000 grit. On really bad rust I may start as rough as 220 and to achieve a high gloss (like a Weatherby finish) I may go as high as 2000 grit! The sanding method gives you surprising control. The rougher the paper, the faster you cut rust and pitting, but the duller the resuling finish. The finer the paper, the shinier the metal gets. I sometimes even wear out the paper to achieve a "scratch free" look.
If you use a buffing wheel, remember that the jewelers rouge on the wheel is held together with wax. You are packing the pores of the metal with wax and then sealing them in by closing those pores when you shine the metal. So after buffing, warm the metal and degrease it again.
Steel Wool has oil in it. So after using steel wool, also wipe the metal down with a degreaser.
To touch up Parkerizing, match the texture by laying a piece of 220 grit sandpaper on the shiny spot and either tapping with a mallet, or better yet, rolling a metal bar over the paper as if it were a "rolling pin". This presses the "bite" of the paper into the metal, making it look "glass-beaded" and results in getting a "matte" or dull finish. The coarser the paper, the duller the finish. If your first attempt isn't precisely matching, switch to a coarser or finer paper as needed and roll it in again! You are never stuck. Just change the grit until the white area looks like the blued area (using the overhead light reflection technique described above). When it passes the "eye-test" while still white, it will pass it as well, after it is blued. Entire firearms can be made to look parkerized by running them through a sand-blaster or glass-beader until the texture is a consistent satin, or matte finish. Now blue it and it looks like it was parkerized,
It is often more valuable to RESTORE a firearm than to repair it. If I have a newer gun and accidentally scratch it on a barbed wire fence while out hunting, I will invariably sand out the scratch, polish the metal to the appropriate shine, and then blue it. I now have my "new" gun back!
But what if Grandpa gives you a firearm that he toted through the woods for 50 years, and it has rust, scrapes, and worn areas from use? If you make it look new - you have destroyed it's collector and/or intrinsic value. Approach this with an extra step in the bluing process that is dependent on texture. Degrease the metal, and then instead of fixing the texture - use fine (#0000 steel wool) dipped into the bluing and scrub the metal with it. Between the bluing (mild-acid) and the steel wool (mild abrasion) the rust will start dissolving, without damaging the existing blue. This will remove light to medium rust and usually reduces the heavy rust even it it doesn't completely remove it. Now blue the metal "as-is". You end up with no rust (or greatly reduced rust) and though the firearm is darker, it has retained its texture. Where it was dull - it still is. Where it was smooth or shiny - it still is. It looks like an old gun in great shape for its age. (That is highly preferrable to a "repaired" look!) The advantage to restoring rather than repairing is that if you are not happy with the results - you can always go further. What you can't do is make it look perfect, and then change your mind later!
Surprisingly, it is often smarter to restore even more modern firearms. As an example, you have a hand-gun that you carry regularly. It now has some holster-wear on it. If you look closely, it is nearly certain that the texture in the worn areas are slightly different than the good bluing. If you fix the texture and blue the gun to perfection, a gunsmith looking at it (knowing that you have carried it for years) may think to himself "Hmmmm? I wonder how bad it was before he "fixed" it" He now thinks of the gun as a "shooter" and it's value drops. If however, you blue it the way it is, when he looks at it, his thought pattern is more likely to be ... "Well, you can see that it has been carried, but it hasn't worn through the bluing yet. He must take really good care of it" ... and the price goes up. It is often more valuable to restore rather than repair and remember, you can always go a little farther if you are not satisfied with your results.
To repeat: Whatever you make the metal look like (to the millionth of an inch) BEFORE you blue it - that is what it will look like AFTER you blue it.