leading

In article , john B. writes snip

That would have been 'killed spirits of salts'. In other words; dilute hydrochloric acid which has had zinc dissolved in it. It is an excellent flux but must be thoroughly washed off the work after soldering as it is corrosive. In the UK 'Bakers Fluid' flux is much the same thing. -don't leave the bottle near any tools etc. as the vapour will cause corrosion.

There are special epoxy fillers for powder-coating - temperature- resistant and electrically conductive.

I'd leave the dings and just coat over them; if they are big enough to be a structural problem you want to notice that, not hide it.

And big enough can be quite small - think crack-initiation...

Something else you might consider is baking-soda blasting.

It is not something to think about. The only place in that part of the country that does powder coating used glass beads - take it or leave it.

I admit that the dings are really too small the worry about but a couple are, for instance, right in the middle of the top tube and they sort of stick out like a sore thumb from the shinny polished look of the rest of that tube.

None of them are serious enough to result in a strength problem but after exerting considerable efforts I want it to be purty :-)

I wonder what panel beaters used back in the day of leading body dents in the front wing?

The same thing -- zinc chloride flux. And their smoothing paddles were coated with beeswax, which protected the wooden paddles and also acted as a flux.

Be aware that ordinary electrical solders are not good for body work, or any work in which you need a pasty temperature range in order to work the lead. Body solders have a long pasty range that allows you to smooth them into shape with a paddle or other tool. Electrical solders have an abrupt melting point and they're nearly impossible to work with in this type of work. They just go "slump" and run off.

Zinc chloride flux. You can buy it already mixed, or make your own. Get some muriatic acid (dilute hydrochloric) from a building supply place, where they sell it for etching concrete. Dilute it with an equal amount of water. Muriatic usually is sold as 31.45% hydrochloric; you want around 15%.

Drop in some small pieces of zinc, letting the solution work until it stops bubbling. Do this out of doors or you'll rust any steel in the vicinity from the acid vapor that goes off with the hydrogen bubbles. Then add some more zinc until the solution is saturated. Leave it overnight. It's standard practice to leave a few zinc pieces in the solution to be sure it's saturated.

Zinc pieces can be obtained from the shell of a carbon-zinc battery, the cheapos sold in discount stores.

FWIW, I personally would not apply soldering heat to the middle of a bicycle frame tube. Unless it's a real cheap, heavy frame, it depends on the hardness obtained from drawing the tube for strength, and it's easily weakened with heat. This is less of a problem at the tube ends, which is how they get away with brazing the frames together.

As you say a "lugged" frame built with butted tubes has already been heated to a red heat in order to braze the tubes, but even the best frame makers seem quite happy to braze fittings anywhere on the tubes. Braze-ons for bottle mounts on the down tube, brake posts on the forks and seat stays, cable guides on the top tube and shifter mounts on the down tube, to name a few, and all brazed at a red heat. My thoughts in using lead was that it could be done at quite a bit lower temperature

- sort of fail safe thinking :-).

I was sorta aware that body lead was different then solder but since I'm just filling small blemishes - think a 30 year old bike - and I had hoped that a bit of flux and a glob of solder and a little filing and polishing and I'd have a smooth tube. I hope :-)

That's true. I've seen some discussion about it, including speculation about the integrity of a frame with those features, but I've not seen any actual test results.

Note that the loads on the down tube are mostly compressive; cable guides are (or were) typically located near the ends of tubes, where bending loads are low; and so on. Also, the two real racing bikes I have owned -- a Frejus road bike in double-butted Reynolds 531, and a Cinelli track bike in Columbus tube -- had no such brazed fittings on main frame tubes.

Bikes in normal use are not stressed enough to test the actual strength of the tubing. They're generally well within the elastic range, because a frame that flexes a lot, in racing, at least, is a very inefficient frame. I owned one, which I kept for about two weeks.

So I'm not saying that you're courting disaster to heat a frame tube in the middle. If it's straight carbon tubing, even soldering heat will seriously compromise the strength. But it's very rare that any bike in normal use would ever approach the strength limits of the tube. Consider, though, that enough local load was once applied to your bike to create those dents.

Good luck. If you don't have a body-shop supply where you can buy body solder, plumber's wiping solder is almost the same thing. But I doubt if any of that has been sold for at least 30 years. d8-)

Let us know how it works out.

50/50 has some mush. Avoid 63% - eutectic or near eutectic. If you want to fuss about it, melt some solder and add more lead to it.

Ha! That's what I considered suggesting. I've done it in a pinch. However, it sounds like John is aware of the issues and that he wants a simple, straightforward solution. Between giving the recipe for zinc chloride flux and mixing one's own solder, I was afraid I'd fallen into the RCM trap of making a new avocation out of a quick fix-it job.

The flux question has been addressed, and you should be able to locate small containers of plumber or general purpose flux at many retail hardware sections, or a welding supply area in a place like Farm-Fleet or Tractor Supply Co. I found a water-soluable zinc chloride paste at TSC which isn't greasy like the more common vaseline-based flux.

When dings/small dents occur in crowned surfaces, there are often raised areas/high spots adjacent to the ding. With thin tubing, attempts to level the high spots can present a problem of weakening the tubing wall.

If high spots are present, they can be seen by checking the surface with a straight edge.

An option might be to cover the damaged area with a vinyl graphic design sticker or similar item, rather than potentially weakening the tubing by attempting to level the surface.

Greetings John, If you want to repair these dings with solder I would say use lead free plumbing solder and paste flux. The paste flux must contain zinc chloride. I think all paste plumbing solder does. The lead free solder is pretty easy to work with and the left over can still be used for plumbing. If you need to build up deep areas use a pointed flame that heats only a small area. This way you only heat up a small area enough for the solder to flow aznd can add as much solder as you need without melting all the surrounding solder. Eric

Use LabMetal:

Certainly heating a stressed tube could have an effect but I doubt that in the case of a bicycle frame it is a major problem. Given that in metal bikes (can't speak for fiber) the main problem is stiffness it is likely that tubes either thick enough or large enough in diameter to be stiff enough may well be able to stand a little heat in the middle and the more modern frames are likely built of something like Reynolds 631 or better that are air hardening.

The frame makers who have been building for many years are not reluctant to add braze-ons where you want them.

Actually, the down tube - runs from the head tube to the bottom bracket - is the most highly stressed tube in the frame - in tension. Think of a really fit racer mashing down on the pedals while pulling up on the handle bars to beat Lance Armstrong up a hill :-)

I suggest that while your racing bikes may not have had braze-ons most road racing bikes do. At least for a water bottle on the seat tube and more then likely for cable guides on the top, down, and R.H. chain stay tubes.

Reynolds 531 has been around for 100 years or more and certainly has had an effect on the better grade of bicycle frames. I doubt that many medium or high priced frames still exist that were built with straight carbon steel tubes.

I live in a country that uses plastic pile exclusively for plumbing. I doubt that one could find a stick of plumbing solder in the whole country :-)

Appreciate it but as (I think) I mentioned in another post, this is not an effort to fill dents it is mainly that small dings and scrapes that a bicycle collects in 30 years of use that I'm contemplating.

I live in Thailand. No Farm-Fleet or Tractor Supply here :-)

True.

Or by running a finger over the damage.

Yes. But these are very small scratch sort of damage due to the 30 years that this frame had been ridden. Nothing major and the effort is mainly to get a really, really, good looking finish. One does not want to be asked "who built it?" and have to reply "me!" if it looks horrible :-)

My powder coating guys tell me that they cure at around 200 degrees(C) minimum.

I'm not even sure that they have lead free solder over here in Thailand. It is amazing what people can get along without. Plumbing? All blue plastic these days.

But your comments about paste flux is appreciated as I can get that and the current scheme is to try paste flux and solid solder if I can find any of that and any solder if I can't. As the frame is in a frame vise when working on it so I can orient the repair spot up and hope that the solder doesn't run off faster then I can apply it :-)

Whoops! I forgot the terms. It's only been about 45 years. Yes, that tube is certainly loaded in tension.

It wasn't clear what type of bike you were talking about. My 30-year-old bike, which is one of my two general-purpose road bikes and which was medium-priced (Schwinn Super LeTour), is 1020 straight carbon, which surprised me at the time.

Anyway, it wasn't clear.