Paint is not sticking to brass

[OldEastRR]

If there's a Construction forum I guess this thread would go there. But I can't get any paint or primer to stick flawlessly to brass car sides. I thought at first just spraying acrylic on metal wouldn't work so I bought some enamel primer to spray on first but still little chips of the final coat flake off -- and not just at the edges.
I clean the brass with MEK, then alcohol, and if it looks a little dull before I use the chemicals I use steel wool to burnish the metal. I'm going to try baking on the primer next but will that help the paint bind permanently to the brass? What's the trick? Do I need to bake on 7 or 8 coats of paint, like for car finishes?

[mmagliaro]

I haven't painted a lot of brass, or metal engines or cars, for that matter.  But on the few I've done, the scheme that works best
is to soak the body in vinegar for 10 minutes or so, let it dry, airbrush it, and then bake it.
I used Scalecoat 1 for this, and they provide instructions on the temperature and duration to bake it.



On the few I did like this, the paint really held on there like iron. 

[OldEastRR]

Vinegar, huh? I might as well try.

[peteski]

What exactly do you do to cause the paint to flake off?  Does it do it on its own, or you scratch it off with your fingernail?  Is the painted surface smooth and even?

Specifically, what brand of enamel primer are you using, and how long do you let it dry?  Could you be spraying it on too dry?

Vinegar is a mild acid - it simply etches the brass surface.

[rsn48]

I have taken this off the old Atlas forum, Note I did not write it, Dean whoever he is did:

Lou, you will find many suggestions on this topic and I will chime in also.

Having painted several number of brass models, most if not all come with a clear coating on the model, some say remove it others will say use it as a base for primer. You can go either way.

Now the fun begins, disassemble the model and remove ALL plastic parts. Suggestion here, if you have some of those little plastic bags detail parts come in they are handy for placing screws, plastic parts etc. in them, label with a marker and masking tape so you know what is what and goes where. "If" you have any flaking of this clear covering, tarnish, corrosion, or discoloration, I have had to microblasted the brass components in my abrasvie blasting booth to remove these stains off the model. This process will also "etch" the surface giving it some tooth for the primer to adhere to. Now is a good time to resolder any loose parts back onto the model. After this process wash the model in a hot soapy detergent bath, I scrub all the parts with an old tooth brush to get in the cracks and crevices, scrub all surfaces inside and out. Dont use any detergent with an additive - scent or no-spotting action as these will leave a residue on the surface of the model that primer/paint may not stick to. After the wash, rinse with demineralized water since this will not leave any water spots. Next I do an alcohol rinse to remove any of the remaing water, at this point I have some lint free cotton gloves that I use so I do not get any finger oils on the model. Using my airbrush I kick up the psi to aroun 40-50 to blow various nooks and crevices of any remaining water. Next I use a hairdryer blow dry the model. Now you model is clean and you are ready for the primer. Paint according to the paint instructions and thin accordingly. Remember several light coats are easier to work with than one heavy one. I personally bake all my brass models in the overn at 150 degress F, for about an hour. Let cool for at least 24 hours, keeping the model in a dust free area. After baking the primer next apply the first color, the bake again at 150 for an hour. Next I usually wait 3-5 days for the paint to cure, giving it the "sniff" test to see if I still can smell the solvents in the paint (you will have some). Now you are ready to apply your next color of paint, follow the same procedure for baking. If the model requires another color or several colors, repeat the necessary steps in between taping, painting and baking. Always when handling the painted model us a cotton or rubber glove to avoid any finger prints on the model, somehow paint doesn't really like to stick to those (speaking from experience here).

It can be along process to do but well worth the effort to have a nice clean and professionallly painted model. After all the painting is said and done, you can decal, clear coat and weather accordingly. I hope this helps you out, if you need further information let me know always glad to help. BTW besides the surface prep, painting a brass model is very similar to painting plastic, just no baking after painting!

dean

Santa Fe all the Way....Middle Division Emporia Sub Circa 1976 - 1986
Hoping someone someday will make a quality ACF 4600

[havingfuntoo]

You can also try washing the brass sides in dish washing detergent first to remove any grease and oil then soaking in the vinegar or phosphoric acid.

Use a diluted brew so you do not damage the surface and remember do not turn your back on it, when you are not watching things happen. After etching, rinse with warm water and leave to dry before painting. If you heat up the vinegar or the phosphoric acid to about 60 deg C you will speed up the reaction considerably but I would not go any hotter.   

[Kisatchie]

The little bit of brass I've painted was with Scalecoat (not Scalecoat II) paint. It went on smooth, and after "baking" in my 100+ degree garage for a couple of days, it was bulletproof. I didn't soak the brass in vinegar because I hadn't heard about doing that.


Hmm... safety warning
for the day: don't soak
termites in vinegar...

[JMaurer1]

I always did the clean/paint/bake method myself and never had ANY problems. One other thing to be careful with is that some paint list 'primer' as a color, not a type of paint. 

[mmagliaro]

Here are the instructions for Scalecoat I;
http://www.weavermodels.com/page12.html

They don't mention the vinegar, but I was sure I originally got that idea from a little instruction sheet they
used to package with their paint.

Anyway, yes, vinegar cleans and etches the surface so the paint sticks better.  The baking really makes a huge difference.
You can only do this with the Scalecoat 1, solvent-based paint.

[peteski]

While these are all helpful hints I feel that at this time we still don't have enough info about the actual problem.  That is why I asked those additional questions.  It will be interesting to find out more about the problem in order to hopefully provide a good solution (or find what the cause it).

[mmagliaro]

That's a good point.
OldEastRR:  In the initial question, you mentioned that you are doing this by first spraying with an "enamel primer"
after cleaning with MEK and alcohol.

Tell us what this "enamel primer" is, exactly.   And remember, MEK and alcohol will certainly clean any dirt or oils off the metal,
but won't etch it, and I think that's an important step.    There are also "self etching" primers.

Personally, I never used primer at all.  Just clean, vinegar, Scalecoat 1, and bake.

[robert3985]

I've painted a LOT of brass, including Brass Car Sides.  The trick isn't etching the surface, because good paints, such as Scalecoat don't have a problem adhering to CLEAN brass.

You don't need to "etch" or sand plastic when you paint it with enamel or acrylic paints, and the same painting principles apply to metals, except metals can oxidize much more than plastics.  Etching with vinegar or phosphoric acid isn't necessary for paint to stick to brass.

What is necessary is to make sure your brass model is "bright"...meaning that it's not overly oxidized.  You can use fine steel wool or, you can dunk it in an ultrasonic cleaner for half an hour which is what I do on my models that are all metal.

I've never steel-wooled my brass car sides as they are usually already bright.

The brass also has to be free of oils and greases.  The plastic parts require this too.

On Brass Car Sides, I dunk the sides for half an hour in my ultrasonic cleaner before I start assembling them to the plastic core.  Of course, during construction, they get fingerprints and oils on them, so I have to remove those oils with something that won't attack the plastic or what's gluing them to the core kit.  This is a two-step process, because certain oils respond to detergents, and others respond to mineral spirits of different kinds such as alcohol or, my favorite...Bestine.

I wash the cars in a warm, mild water/dishwashing (I use plain, old "Dawn") detergent first, using a soft toothbrush to get into the nooks and crannies...with a very light touch so I don't bend or remove any detail parts.  After I'm done scrubbing and the model is still in the bowl, I dry my hands and put on fresh nitrile gloves, then rinse the model with warm water thoroughly.

I then blow the model dry using my compressor air, which is properly filtered against both oil and moisture...this goes quickly...no more than five minutes, but I live in a dry climate. You need to hold it securely as I've had a model ready to paint shoot off onto the floor when I've had the pressure turned up too high.  The nitrile gloves assist in giving you a good grip.

Then, at my spray booth with the fan going, and with a 1" wide, soft artist's brush, I wet it with Bestine and brush all surfaces of my model with a generous amount of the solvent, which does not affect plastic whatsoever, but is a powerful oil/grease/wax remover.  I make sure the Bestine has evaporated by blowing the model with my old 1200 watt hair drier set on "medium"...this process goes very quickly as Bestine is extremely volatile.

I use a holder (not my fingers) to paint my assemblies, which can be a smooth hemostat, a piece of masking tape with the part stuck to that...then stuck to a dowel, or a paper towel tube stuck up inside the car body without the roof or chassis.  Whatever you use, these "holders" have to be dust and debris free or the loose stuff will inevitably come loose and stick to your model's wet paint.

Time to paint.  I have already made sure my paints are good, and I use a small battery powered propeller-like "mixer" to mix the pigments in my paints.  I don't shake them anymore, and this has greatly improved the paint quality and consistency.

I don't do anything out of the ordinary when painting and most models I don't prime them, unless I'm spraying Armour Yellow, then I'll apply a thin, smooth coat of gray primer to make the Armour Yellow fully opaque.  Truthfully, now that I'm mixing my paints rather than shaking them, my Scalecoat yellows have been covering much better.

Using either lacquer or enamel based paints and with models that are completely brass (no plastic) I bake them at 135 degrees for ten minutes in my small toaster oven that has been pre-heated to 135 degrees, using an oven thermometer inside the oven I can see through the glass door to confirm the temperature.  I don't want it too hot, because it darkens the paint...especially Armour Yellow.  After they've baked, I take them out and let them air cool until I can handle them with my nitrile gloved fingers, then I blow compressed air on them to speed up the process.  They're ready now for masking and another color.

On plastic models or composite models such as what you get when using brass car sides, with enamel paints, I let them air dry for a day or two...until the smell is mostly gone.  I've found that the smell doesn't have to be completely gone, and usually a couple of days at room temperature works okay between colors.  To speed things up in the summer, put them in the garage or in your car's trunk...or in a box in the back seat, but don't let direct sunlight hit them.

However, with acrylic paints, I use the hair drier after the coat has become non-runny, blowing both warm and hot air on the model.  This cures the acrylic good enough in about ten minutes of hot air to mask and apply another color. 

Using these methods over the last thirty years or so both at home and as an industrial model builder for the aerospace industry, I haven't had any failures or problems with paint adhering.

However, I make sure that I properly filter my compressed air against both oil and moisture, so I'm not blowing any out during airbrushing or just blowing my models dry.  I also use common adhesives for gluing my models together like CA (and Accelerator), epoxies, Walthers Goo, and Tenax.

I also wash my hands a lot during model construction and my hands are not naturally sweaty or oily.  If I get up and leave to make lunch or dinner, I always wash my hands before starting on the project again.  If I scratch my nose or forehead...I'll get up and wash my hands before touching my model again.

I also never use paints that don't appear to be 100% for fine finishes on models I've spent a lot of hours on.   If I don't have a spare bottle, I'll just wait until I get to my LHS, or order what I need and not worry about it until they arrive.  I always get at least two bottles of the colors I use regularly, so I'll always have a fresh bottle available if one goes bad.

I've found that painting metals is easier than painting plastics, and that paints stick to it much better than paints to plastic.  I'm almost 100% sure that your problem involves either bad paint, or more likely, a grease, wax or oil that your cleaning hasn't removed.

[peteski]

That is quite a writeup Robert - do you have bunch of these helpful hints stored somewhere and you just copy/paste the same info as-needed?     Seriously though - it is a very good advice.

But the original question contained this sentence: I clean the brass with MEK, then alcohol, and if it looks a little dull before I use the chemicals I use steel wool to burnish the metal.

That is why I asked for additional information before flooding him with sage advice which he might already be familiar with.    I suspect the problem might be with the paint itself or with the application method.

[mmagliaro]

Bob,

Thank you for a very thorough and complete write-up, based on lots of actual experience!
I suspect now that using vinegar merely helped to dissolve oils and dirt from the surface.

[robert3985]

That is quite a writeup Robert - do you have bunch of these helpful hints stored somewhere and you just copy/paste the same info as-needed?     Seriously though - it is a very good advice.

But the original question contained this sentence: I clean the brass with MEK, then alcohol, and if it looks a little dull before I use the chemicals I use steel wool to burnish the metal.

That is why I asked for additional information before flooding him with sage advice which he might already be familiar with.    I suspect the problem might be with the paint itself or with the application method.

I just write from experience.  Luckily, I'm a 100+ wpm typist.

Yes Peter, I did see that he cleans with MEK and alcohol.  However, there are some surface contaminants that either of these don't touch, but simple detergent and warm water will take right off.  I included a step by step of exactly what I do so that the OP could examine his techniques and possibly see something that he doesn't do that would solve his problem.

Personally, I'm not too big on alcohol as a degreaser, and it certainly does virtually nothing to remove waxy deposits...hence my recommendation for Bestine which is a wonderful and powerful degreaser as well as de-waxer and is readily available at your local Michaels.

I haven't used MEK on plastic as a degreaser since it will attack and dissolve polystyrene, ABS, Acrylic, Butyrate, both low and high density polyethelyne, polycarbonate, Delrin and several other plastics.  It is a major component in most plastic cements.  If the OP has actually used MEK as a degreaser, he must have done it before gluing on his brass car sides, because it would have minimally attacked the core kit if he'd used it after the sides were glued on.  When I was working at Thiokol and Battelle Labs as a model maker, we used pure MEK to weld our plastic constructions together and we dissolved Styrene in it to make "thick putty" to both fill cracks and bond plastic pieces simultaneously.  However, it does have an excellent reputation as a degreaser for metals and is often used in industry to clean resin and adhesive residue from metal surfaces as well as grease and oil.

He may also be getting contaminants through his airbrush, which is not unheard of.  That is why I emphasized oil and water filtration on the compressor, although an oil-less compressor or C02 will eliminate oil contamination, water condensation could still be a problem, especially if shooting lacquer or enamel paints, and could contaminate the paint enough so that it won't stick.  I always recommend a Paasche in-line water trap on all airbrush hoses.

Writing the advice in a logical format keeps it from being a "flood", and we all know that you are full of sage advice which we all appreciate!   

Hopefully, me putting my process down step-by-step will quickly solve the OP's problem without him having to respond to a batch of questions over several days.