Bluing without blue remove?

What happens if I try to reblue the slide of my m41 without removing the original blue?

I don´t want to damage the markings or the sharp edges..

What bluing do you intend to use? If one of the Brownell top rated bluing like Oxpho Blue, you can use it right over existing blue without affecting the factory finish, while darkening the worn areas. Multiple coats may be needed to get to the color you want.

I will use the with the classic formula of nitrate of potassium, caustic soda and water...

Although the finish may be somewhat blotchy, if the original finish is a nitrate blue and that is what you use it will do no harm. Remember the quality of any finish is dependent on the quality of the polishing done to prepare the surface, and obviously you won't be polishing the gun. I predict you will be dis-pleased with the results, but at least you can still strip, polish, and try again if this happens.

I was told by a gunsmith that he didnt polish off all the old blue when he did spot work, just got the polish similar and they blued OK. All i saw that he did were fine when so done on repair work. I had an old school gunsmith blue the bolt of my 1903 once, he gave me a price that included polishing it, i asked how much without the polish, just throw it in the tank. He said it wouldnt work, that he was shown that they always had to be polished to white to blue. i told him to do it without the polish and id pay him the same whether it went OK or not. It went OK. He was quite surprised, saying hed never have believed it if he hadnt seen it.

Most of the problem with a refinish isnt that its refinished, but that its over polished by people that really dont know how to do it that well, and they usually use the wrong tools, like way too small of wheels. The factory doesnt ruin the edges and lettering, and they use wheels that are around 16" diameter, not the comparatively small things most gun refinishers (notice I didnt say "gunsmiths") use. Still, its not required to remove the old blue to hot blue a part. Ive seen it done many times.

You don't have to completely strip old blue. It really depends on how nice the finish is and what you are trying to touch up.
We used to re-blue many, many guns and gun parts in hot salt blue that were mearly worn. This to kick the appearance of them back up an appreciable amt on an otherwise pristine looking gun.
Sometimes a little careful polishing was necessary in the worn areas, often not. Depends on what is there.
Scratches, scrapes, dents, dings and pits all have to go of course, so blending in those areas to the original polish that exists under that factory blue is a necessity.
That takes some experience to do.
Not matching it in, it will show like a bad spray can touch up on that old Ford you used to have.

Sometimes the finished product out of the bluing tank will take on a different color hue than the original. Slight difference, but noticeable if you were to do one part and not another on the same gun.
Even using the same mfg'rs bluing salts (Brownells or DuLite are the most common),,there are too many variables in the process to expect ones bluing set up to completely match anothers.

Redoing a sideplate on the S&W and then placing it back on the revolver is a good example. Sometimes it'll match just right. Often there is a slight color difference even with the same process being used. Sometimes it takes a look-see in bright sunlight to see it,,that being the best way to judge.

So yes you can re-blue over old blue if the old stuff is in good shape to begin with. I usually recommend doing the whole gun or atleast the major part(s) so as not to show up a big color difference when reassembled.
Small parts such as screws, sights, latches, ect can often be and are a bit off in color from the main parts anyway in hot salt blue.

You can reblue rust bluing over old rust blue also. Same things to look for. It;s best to scrub the old rust blue down a bit before the first pass to rough up the surface and break through the usually old sealed coating of oil and 'stuff' imbedded into the usually matted rust blued surface.

(Over) Polishing is to blame for most bad rebluing jobs, not the bluing itself.

I has a Colt 1911 at my gunsmiths to have Millet F&R sights installed. I got it back and a couple days later I noticed several fine vertical scratches on either side of the slide. No white metal, the scratches were blued. I know every nick and scratch on my guns and they weren’t there when it went in. I always believed that he dunked it to cover his butt.

When I purchased my 29-3, there was a scratch on the left side of the barrel. Not deep enough to score the metal, but the blue was gon there.
Fixed it with Blue Wonder. Not a true "cold blue" as heat is necessary.
Followed the directions to the letter.
Did not affect the gun's blue, but the scratch totally disappeared.

And stayed disappeared through years of use and cleaning.

I would use Brownells Oxpho blue for something like that. Clean the parts with a good degreaser and warm the part with a heat gun or blow dryer. Don't get it hot but warm to touch. Use the Oxpho with long strokes going all the way down. Let it set for 1 minute and clean with a dry rag. Use 0000 steel or better yet bronze wool. Don't rub too hard, just enough to brighten the bluing. I do at least 6 coats. after the bluing looks dark all over coat it with a good gun oil.