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Fast Tracks :: View topic - Acid vs. Low Acid Flux - Which Is Better?
Fast Tracks

Acid vs. Low Acid Flux - Which Is Better?
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Author:  Ron Warris [ Fri Jun 05, 2009 3:27 pm ]
Post subject:  Acid vs. Low Acid Flux - Which Is Better?

It seems that nothing else is more polarizing and generates more passionate debate then our recommendation to customers to use acid based flux for soldering trackwork. The topic comes up regularly both in this and other modeling forums with experts lined up on both sides of the acid vs. no (low) acid flux issue.

On one side we have many people saying that you should never use acid based flux. Period

And then you have the other side (namely us, Fast Tracks) recommending that you should always use acid based flux for soldering trackwork.

So what is the story here? Why do we recommend acid based flux, and others recommend against it? The fact is, both answers are right. Both flux types will work just fine. But there are important reasons why we recommend acid based flux over low acid flux for soldering trackwork.

To try to explain why we advocate the use of acid flux and answer the questions that keep coming up about this, we have produced the following list of Frequently Asked Questions (FAQ).

Why do you recommend acid based flux?

Acid based flux will always give you better results with less effort and is highly recommended for folks who are new to soldering. Even if you are an experienced solderer, we still recommend you use acid based flux as you will always get consistent, high quality results.

Why do others recommend low-acid based flux?

It is felt by some people that using an acid based flux will result in corrosion problems over time. However we have found that as long as the finished trackwork is properly cleaned, this is not an issue. We have trackwork that is over 15 years old that was soldered with acid based flux and not cleaned that is still as solid as the day it was made. (Although we do of course very much recommend that you always clean your trackwork after soldering.)

I was always told to never use acid flux for wiring and electronics. Why is it OK for soldering trackwork?

This advice is correct, acid based flux should never be used for soldering wire joints or electronic components. Soldering trackwork on the other hand is more akin to soldering plumbing joints than electrical joints. You are more interested in producing mechanically sound joints than electrically conductive ones. And like plumbing, acid based flux is recommended for producing robust, mechanically strong joints.

Won't the acid corrode the trackwork over time?

As long as the finished trackwork is properly cleaned, the trackwork will not corrode over time.

How should I clean trackwork soldered with acid based flux?

We recommend that the finished trackwork be cleaned with a wire brush. You will find complete illustrated instructions on pages 42 & 43 of our soldering document that is included with every order that we ship, or on our website here:


Should I use baking soda to neutralize the acid?

You can, but it is not really necessary. If you are concerned about corrosion over time it is an extra step that you can take that will completely neutralize any remaining acid after brushing the trackwork with a wire brush.

My solder joints look fine and I just use low acid flux. Why do you still recommend acid based flux?

We have seen hundreds of examples of finished trackwork from our customers, and while many people believe they are getting good results with low-acid flux, we have found that many examples are not as good as they could or should be. A sound joint will have solder that has completely liquefied and flowed between the rail and PC Board tie. Getting the solder to a fully liquefied state is usually not a problem, but getting it to 'flow' in between the rail and ties takes a bit more practice. Acid based flux makes it considerably easier to get the solder to flow, resulting in a solder joint that is very smooth and flat.

Here is a good example of sound solder joints.


If you study this image carefully you will notice that the solder just 'wets' the tops of the ties, gently rising up and over the bottom of the rail. The joints are smooth and void free. These types of joints are very possible with low acid solder flux, but are much harder to produce. Acid based flux on the other hand makes it much easier - and is quite safe over the long term.

So which is better? Acid or no-acid flux?

Both will work fine. If you are new to soldering, we recommend acid based flux. If you are an experienced solder, then using low-acid flux will do the job, although you will always get better and more consistent results with acid based flux.

Our objective is to provide the best tools and advice to our customers so that they are able to produce robust trackwork that works reliably over time. And sound, well formed solder joints are a key part of that objective. Acid based flux increases your probability of success, which is fundamentally why we recommend it's use. Corrosion issues over time on the other hand can easily be mitigated with simple cleaning techniques.

Fast Tracks

Author:  bicknell [ Sun Jul 12, 2009 10:10 pm ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

I don't think your advice to use acid flux is bad, but I would prefer to have to do excessive cleaning so I've tried some good "no clean" flux in N scale. I also like it because the Kester stuff comes in convenient pen form.

What I wonder from my experiences (nearly identical results with both) is if this isn't somewhat scale specific. It makes sense to me that with larger scale track, and larger PC Board ties there would be more advantage to acid based flux. As you get smaller and smaller, the advantage diminishes, to the point where with N and Z it is an extremely small advantage.

I only have an N scale fixture, so I can't offer direct evidence. However, it makes sense to me on several levels. Can those who have made turnouts in multiple scales comment?

Author:  emccamey [ Mon Jul 13, 2009 9:24 am ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

I've been of the vocal opinion that acid based flux is a crutch for poor soldering technique. Cleaning the parts well and using a non-corrosive flux that does not REQUIRE a through cleaning is a far cry better approach. Using the water based liquid fluxes allows the suitability of in place soldering and after construction adjustments that do occasionally become necessary. Those who site construct in place using the flex tie bases and other custom finishing should not consider acid flux.

I use acid fluxes when I construct a copper live steam boiler in the MUCH larger scales - but that also is followed by a thorough bath of pickling cleaning process. Scale model trackwork has electrical control and a safe non-acid technique is very highly recommended.


Author:  sambear [ Mon Dec 28, 2009 11:39 am ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

I have tried both Acid and Non-Acid Flux.

Both worked well to solder rails to the PC board ties but I definitely prefer Non-Acid Flux (SuperSafe® Gel is the one I used) because it is easier to wash the work clean after the soldering is done.

Though I washed the acid-soldered #6 turnouts (I may have done a poor job of that by not using a wire brush), all but a couple of them have black PC Board ties now. The #10 turnouts were soldered with SuperSafe and then washed; all still look clean a year later.

Hope this is of some help...

Author:  sd80mac [ Thu Feb 11, 2010 11:44 am ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

I actually got better results, meaning much stronger solder joints using the recommended acid flux. I did clean the track following soldering, but its only been a day since, so I'll see how it looks after a week. I'm confident it will be okay.


Author:  shap [ Thu May 22, 2014 12:21 am ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

Here in the U.S., the Kester flux is slowly getting harder to find. As U.S. piping standards have eliminated lead, the use cases for Kester flux are slowly fading away. Kester itself now identifies the product as obsolete.

I've been having pretty good luck with Oatey No. 95 Lead-free tinning flux.

Author:  Zoinks007 [ Sun Mar 13, 2016 10:21 am ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

The discussion here is a good one. At the risk of sticking my nose in where it doesn't belong...

Being a computer/electrical engineer, I've soldered many circuits. But as a homeowner I've soldered many pipes. The goals and needs of both are vastly different, although both share a need for mechanical soundness.

Seems to me Ron and Tim have a dilemma. They build jigs for turnouts and sell them to newbs like me. Then the newbs cut ties and rails, shove them into the templates, and bath everything in a coat of too much solder and flux. When it doesn't work, because we've never soldered before, who do we call? Ron and Tim. So they have to post-mortem and remotely "debug" someone else's work. If I were them I'd recommend the approach that's reasonable and works nearly universally.

Speaking for myself, I'd never heard of Kester. I use Oatey for my pipes (last project was a pneumatic system for my garage, so good solder connections were mandatory as was thick-walled copper pipe). But it requires good cleaning, both before and after. I don't care about what it looks like aesthetically, I just want a good solder joint.

Here, similar thing. The acid flux comes with the kit, so I'll give that a try. I'll wire brush to clean, but I'll also wipe with lacquer thinner since I personally am anal about cleaning metals prior to painting. But in the end once painted, I don't see the rails corroding and failing any more than I believe the pneumatic system I put in will corrode and fail.

I certainly don't have the experience Ed has. His left pinky knows more than I'll ever know about hand-laying track. But I do have a lot of soldering experience in many different venues. If I were Ron and Tim, trying to teach someone who's never done soldering to create mechanically sound joints, I'd have to recommend the stronger flux as well just to help make sure my customers were getting good results from the start using my jig. The jig likely wouldn't be the culprit for a poorly-performing turnout, but you can bet the customer would lodge the complaint, fairly or unfairly.

Author:  hoborob [ Sun Mar 13, 2016 1:45 pm ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

Back when I first started building turnouts I asked where to get the acid flux that was suggested in the videos. I was directed to Ace Hardware which carries an Acid flux that is not quite a gel but not a liquid either. The best part of it was that it was water soluble so cleaning it off the turnouts was very easy, just carry the turnout over to the sink with a toothbrush and give it a goo scrubbing. Worked very well. I don't know if this particular flux is still available or not I have not needed any for a while now.

Author:  hoborob [ Sun Mar 13, 2016 1:48 pm ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

I just checked and the flux I got is called Water Flow 2000 and a small container is $5.49 on the Ace Hardware website.

Author:  MikeM [ Sat Mar 19, 2016 11:06 pm ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

A tangential question, which type of flux would you recommend for connecting feeder wires? My apologies if this has been discussed elsewhere.

Author:  hoborob [ Tue Mar 22, 2016 3:10 pm ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

I have not had any problems using the PC Tie connections as the solder is already well adhered to the rail and the tie. As long as the acid flux has been cleaned thoroughly off of the turnout there is no danger of corrosion occurring at the joint/ Make sure that you pretin the wire before attaching it to this connection though. A solder to solder connection is very reliable electrically.

If you wish to use a new location then you may use whatever you favorite flux might be. I have found that the rosin in the solder is normally more than adequate for the job as long as you pretin both the rail and the wire separately. Otherwise you have to use a lot of heat to get the assurance that a good electrical contact has been made between the wire and the rail.

Author:  Zoinks007 [ Tue Mar 29, 2016 8:59 pm ]
Post subject:  Re: Acid vs. Low Acid Flux - Which Is Better?

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