What is the best product to use to touch up a circular spot about the size of this "O" where the bluing has been rubbed off on a blued steel rifle receiver?

Maybe this:
https://www.amazon.com/Birchwood-Casey-Fast-Drying-Fast-Acting-Restoring/dp/B000TTIEY6

I’ve used that blue pen a good bit and it has worked great for me.

Thanks - I'm getting ready to order one.

I've had the best luck with Brownell's Oxpho Blue. I always get the metal hot with an old heat gun first and do it in layers. It won't replicate a deep, lustrous blue on a large spot, but it'll effectively hide small scraps and nicks or a worn crown.

Thanks

Absolutely agree! I use boiling water to preheat the metal, but heat is heat. The BC bluing products come in well short of Oxpho-Blue.

without heat it won't matter what you use. Heat does it best, just warm to the touch. And being super clean is critical. Some may disagree but I used every bluing and it looks good when first applied but doesn't last. Followed instructions to the letter too, until a gunsmith told me about heat. Again, clean steel is critical. I scrub with 000 steel wool with degreaser and rinse scrub with wool.

+1 on Oxpho blue, liquid or creme it's the best cold blue that I've found.

Plus four on Oxpho Blue. More durable than one would think also. I once refinished a Winchester 67 on a dare entirely with Oxpho Blue. It turned out pretty nice if I do say so, and a buddy liked it so much he gave me more than I thought the poor thing was worth so he could give it to his pre-teen son. The kid is now in his early 20's and by all accounts rode that .22 hard but the bluing has held up nicely.

Gary - what does your method look like in terms of steps? I typically degrease, heat, apply, wipe with oil, then repeat the process. I try to apply in small areas at a time and rub in the liquid to get the color as deep as possible.

I’ve had good luck with Birchwood Casey bluing paste. Heat gun on the spot in question, not enough to sizzle the paste, but good and warm. I’ve done larger spots, sometimes 2 or 3 applications with rinse and repeat process is necessary to get a good blended result.

Y'all, I want to touch up a spot literally about the size of the diameter of a #2 pencil lead.

For tiny little spots I apply with a Q-tip.

Thanks

Yep. Also, never contaminate the bluing solution with a dirty q-tip. Always use a clean one when reapplying. Heat the area you want to blue, and use a good bluing solution. I like super blue and Oxpho.

I have seen less than reputable dealers at gun shows using a black Magic Marker

I polish, degrease, heat, apply. Lightly rub out with oil-free 0000 steel wool, re-heat, apply, repeat as necessary until finish is even and color is satisfactory. Number of cycles depends a lot on alloy of the steel. I never oil until all done, then give everything a liberal slathering of oil and let sit overnight (applies to rust bluing projects also).

Small touch ups = Q-tip. Larger areas = small wad of clean white cotton flannel.

Thanks, Gary. I'm trying to recall why I included the oil step each time... I want to say it's because I originally watched a Potterfield video about how to do it and that's what he did. But I honestly don't remember now.

You’re contaminating the bluing if you do that! Once you scrub to prep, then what l do is, just wipe with cool water and degreaser to clean off the oxidation and use a tooth brush to get in small areas. Dry..then reheat and repeat until desired bluing.