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Update: Oct. 16, 2011
I gave this advice to a Flickr user who asked about using Gun Blue on a bike frame. I thought it might be helpful to others considering this finish:
I think gun bluing if a great way to approach frame finishing, but in my particular case things worked out poorly. I used a newbie powder coater who offered me a good price to apply the clear. I think he made a bad choice on which powder to use--like he went with one that is normally a top coat and is insufficiently tough enough to be a single coat. Long story short I got rust ... Bad. After commuting in snowy conditions a couple times the salt slag somehow got under the coat and the rust spread around the BB like a disease.
Now I'm having the frame recoated in a solid color by a good and recommended coater. He told me that the rust wasn't a product of the bluing -- it should have worked well if the original coater had done a better job.
As for bluing tips:
I was bluing a completely stripped frame that was cleaned up with a fine rotary wire brush drill attachment. I went over the frame with a very hi grit sandpaper -- like 800 -- and steel wool.
When applying the blueing agent, work from the bottom up - seems counter intuitive but this minimizes the effect of drips. If drips run down the raw unfinished steel they will appear darker when you hit up that section with its own layer of blue. If your working from the bottom up the drips only hit finished areas. Be quick with a rag to wipe them up and the won't leave a mark.
Do one full tube at a time -- that's a larger area than the Bluing box recommends but just work quickly to get it on evenly as fast as possible. Overlap can leave uneven darkness, so you don't want to have two work sections joining mid-tube. Wipe the blue on in long tube length swipes that end at the welds or lugs. Might consider using something larger than the swab they provide to get it on faster. A small sponge maybe.
Also, that stuff works fast so you can pretty much start cleaning it from the first area you swiped as soon as you finish the last swipe. Better to keep it light and hit it up again than let it sit and get irrevocably dark.
Note to anyone considering applying clear powder coating to a bike frame, gun blued or not: The heat of powder coating darkens raw steel. With one bike I coated clear, I hoped to preserve to look of cold raw steel, but it yellowed when it was coated... it didn't look bad--sort of made it look brassy-- but it's safe to say that you shouldn't expect the clear-coated frame to look exactly the same as id did pre-coating. In the case of this frame, the blue got darker too. Mine even took on a reddish hue behind the blue that was unexpected but, in a way, richer and awesome. Because of this darkening, you might want to leave it a little lighter than you want the finished frame to be. That will also help the texture of thebrushed steel to come through. Too dark and it will just look black after coating.
Last, keep it in a dry area until you are ready to coat. Bluing doesn't do much to ward off rust, and if it does rust you will be in a jam because you can't knock back the rust without removing the finish. I experimented with adding gun oil to keep the frame rust free until I got it to the coater. I think this was a mistake because the oil is the enemy of powder coating. It had to be fired off before the frame could be coated - this can change the tint. And it may have been remnant oil creating voids between the coating and tubing that led to my rust down the line. Regardless, your coater should treat the frame with an astringent to get all hand oils off the frame prior to coating.
Despite the failure of this frame I will definitely try this on future frames. When done well, it can be awesome. Next time I'm just going to be very upfront with the coater that I want a bombproof clear - possibly too coats if that can be done without too much discoloration.
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