Cleaning and Priming metal

[Cleat]

I'm about to clean some of the metal parts for my King of the Mississippi riverboat.  I have an ultrasonic cleaner and degreaser.  I'm wondering if diluted degreaser is the right solution for the job.  I'm also trying to decide if I should use automotive primer or a water based primer; I'll be applying acrylic paint with an airbrush (and details with an artist brush). 

[Roger Pellett]

I have used several techniques to clean brass parts.  I keep a jar full of lacquer thinner on my workbench.  I use those little jars with screw lids that restaurants serve jelly in.  Baby food jars with lids would work too.  I dunk very small parts in these jars, screw the lids on and swish the jar around.  I then take the part out with tweezers and put it on a paper towel to dry.

 

I recently tried cleaning a large part with household scouring powder and hot water. I dipped a wet toothbrush in the scouring powder and scoured the part.  The part was then soaked in hot water.  This worked really well.

 

I also have a cheap crock pot for heating a solution of Sparex and water.  I have tried this approach but don’t see that it offers any advantages over the above two.

 

I would prime the part with an auto body primer or better yet a hobby primer.  The Tamiya primers seem to work well. Acrylic paints will adhere to these.

 

Roger

[Oldsalt1950]

What you use to clean your parts will depend on what they are made from. For brass use isopropyl and then vinegar to lightly etch it. Most parts can be cleaned with simple dish soap and water. You are basically trying to make sure there is no dirt and oil from your fingers on the parts. No real sense in buying an expensive degreaser when you can get away with a simple cleaning solution. Just be sure to not contaminate the parts by using your fingers to pick them up and use them once you have put them in a cleaning solution. Tweezers or gloves are the way to go. I used to work as a Finishing Painter for a now defunct aircraft manufacturer, and we used a highly toxic chemical to clean parts for painting known as MEK , a drop of it would literally such all the oil out of your skin, but the parts were squeaky clean.

 

Jim

[Roger Pellett]

In the case where brass has been soldered, good practice dictates removal of flux, splatter, oxidation, etc.  This will require more aggressive cleaning.  Sparex, a proprietary chemical, in a heated water solution chemically cleans this harder to remove contamination.  I found Sparex to be tricky to use.  Forgetting to turn off the crock pot, I was left with a dried out residue.  The slightest contact with ferrous materials leaves everything with a copper plated surface.  It would probably work better for professional jewelers than for modelers’ occasional use.  I therefore, tried simple scrubbing with common household scouring powder on a wet toothbrush followed by rinsing in hot water.  This worked well for me.

 

Roger

 

 

[Jaager]

I believe the active ingredient in Sparex is hydrosulfuric acid.  It is classified as a weak mineral acid.  This means that its pH (an inverse measure of hydrogen ion concentration) in water is higher than that of a strong mineral acid ( sulfuric, hydrochloric, nitric, hydrofluoric, hydrobromic acids).  All will dissolve metals.  The strong acids just do it more quickly.  It is wise to treat it with respect, pay close attention to the recommended exposure times, and take care with how it is discarded (totally neutralize).  

 

M.E.K.  is still available from hardware retailers according to a search.  It is very dangerous to breath, possibly fatal if the body inhaling it is also metabolizing certain other toxic substances at the same time.  For our uses, acetone is a much less toxic and is an effective substitute. 

[Dr PR]

Acetone is an excellent solvent for resin solder flux. We used to use it for cleaning circuit boards. Now we use water soluble flux that cleans up easily with water and seems to do a better job that resin.

[thibaultron]

What flux is that? I have a large number of HO track switches to build, that will be soldered to PC board ties, during construction. A nice cleanable well working flux would be helpful.

[Roger Pellett]

For the extensive brass fabrication that I am doing for my current project I am using Bakers Fluid.  This is a water thin clear liquid. I apply it with small paint brush.  It apparently has very low surface tension as it flows into tight joints. Solder seems to follow it.

 

I have also learned to “match the solder to the application.”  I have three diameters of solder- .015in, 1/32in, and 1/8in.  For most work, 1/32in seems ideal.  It does it’s job without dropping a big blob that must be cleaned up later. I will be moving on shortly to 150 or so tiny ring bolts for wooden hatches.  For these, I intend to use the .015in.  There are also some applications where solder is built up and then shaped to build a contour.  For this the 1/8in diameter works well.

 

Likewise, I have several soldering irons with different tip size.  The idea is to use the smallest tip that will heat the joint. A tip that is too large can deposit a big blob that will require extensive cleanup or can ruin the work.

 

Roger

[Dr PR]

The liquid flux I am using now is LA-CO Industries N-3 All Purpose Flux. It is a poisonous water solution of hydrochloric acid and zinc chlorides. To clean it just wash with clear water.

 

At work we used a citric acid based flux (I don't recall the brand), but I really liked it because it smelled like orange juice!

 

I'll echo what Roger says about using very thin solder to control the amount being applied.

 

I have soldered electronics circuitry for at least 50 years, and I used resin core solder for much of that. But one day I was watching a newly hired young woman in our assembly area solder some ICs onto boards I had designed. The pin spacing on the quad flat packs was only 0.5 mm (0.020") with 0.005" (0.127 mm) gaps between the pins. Trying to solder them with resin flux was a major headache. I got solder bridges on at least half of the pin pairs, and it took time to clean them up.

 

But this woman was using the citric acid based liquid flux, applying it with a small paint brush across all the pins. She just put a blob of solder on the end of the iron and wiped it across all of the pins on one side in a quick swipe! Do that with resin flux and the entire thing would be a solder bridge. But to my amazement the solder blob flowed over the pins, and as it passed one by one a tiny amount of solder flowed under the pins in the liquid flux. There were no solder bridges and at the end of the pass there was still a blob of solder on the iron tip!! Under the microscope every one of the IC pins had a perfect solder fillet!!! She soldered these 60-80 pin ICs in just a few seconds, and with resin it would take ten minutes or more.

 

The beauty of liquid fluxes is that they flow with capillary action and draw the solder into the joint between even the tightest fitting parts. We just put the soldered boards into an ordinary dish washer (without soap) and ran it through the rinse cycle. For small quantity hobby work just hold the pieces under running water for a few seconds.

 

I am told you can use lemon juice as a flux, but it would contain a lot more contaminants than a pure citric acid solution. It would probably leave some charred organic compound residue.

 

You can use liquid fluxes with ordinary soldering irons or resistance soldering tools. For high powered soldering guns I would use resin core or acid core (depending upon the metals) solders - the liquid flux might boil away too quickly before the solder had flowed completely.

Edited December 13, 2022 by Dr PR

[wefalck]

If the flux is just citric acid, chances are that you have it already in your kitchen as de-scaler for coffee-machines and the likes. Over here in Europe we buy it in 500 g boxes in powder form in supermarkets. Just dissolve the crystals in distilled water (preferably), as the carbonate-hardness of the tap-water would consume some of the acid.

[East Ender]

Like Oldsalt1950, (Jim) mentioned, MEK is probably the best degreaser out there. Just a whiff of it takes me back to working on Navy Military aircraft at Grumman. Use a simple but  appropriate respirator if using for prolonged periods, and wear dishwashing gloves or anything equivalent.  If anyone is making small parts using aluminum, dipping it in Alodine 1200 will give it a light brown color while etching for paint at the same time. Alodine 600, used for electrical bonding,  will give it a golden or brass color. When we used to trim aluminum skins, the edges would be cleaned with MEK then brushed with Alodine 1200 followed by zinc chromate primer. I'd try it and use Alodine 600 if I had some small aluminum parts.  Put them in a jar in with the alodine for about a minute or two, then remove them and dip them in water to stop the etching process. You could probably simulate brass with aluminum if the part is treated correctly.

Edited March 13, 2023 by East Ender
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