Paint is not sticking to brass

[peteski]

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.

I for one enjoy troubleshooting - maybe because that's what I do in my day night job. 

[OldEastRR]

Sorry I was gone so long ... I airbrush with AccuFlex acrylics, but for this custom job involving brass sides attached to stock RTR car bodies I bought a rattle can of interior/exterior enamel primer. To try to remedy the flaking I used primer on the last batch, but didn't bake them as I had cut out and soldered recessed doors onto some of the cars sides and didn't want them melting off. I used MEK to remove the clear Krylon on the sides, then steel wool to burnish them bright.
The paint only comes off in a few little bits here and there on the car sides, usually just from regular handling to put them in their boxes or setting them on the track. For the most part it stays on but these bright brass "chips" stand out pretty clearly.
Since the brass shows through I figure the Accuflex is sticking to the primer, which isn't sticking. And I don't have a dust-free sterile environment to spray or dry painted items in, so there always will be dust in the air.
Max suggested vinegar and that makes sense since I believe they acid bath brass before electroplating it.
I'm also considering brushing vinegar onto places where decals go and letting it sit a while then dry it and place decals as usual. Some decals just don't seem to want to stick to the paint no matter how much decal softener I use.
I primed another batch of sides before I learned about the vinegar, but I did bake this set. Then I primed and baked them  again. It seems like baking makes the paint shrink down and get into every etched line or gap in the brass. I even oversprayed a little but the baking eliminated the excess paint somehow so there's no trace of thick coat sag. I thought I'd filled the seams of the door edges with too much paint so they were obliterated but they reappeared clear and distinct after the baking. Does excess paint boil off?

[bbussey]

For brass passenger car sides, I have painted the sides with Scalecoat, baked, decaled and sealed them before assembling them to the plastic core kit, which also was pre-painted.  For composite models, I used Accu-Paint (now Tru-Color).  And I did make sure the models were clean before a vinegar soak and let them air-dry.

[havingfuntoo]

Vinegar will not remove oils or grease from the brass, you need to degrease first, regardless of what you use to do it. I stress, the easiest way is to use dish washing liquid, it is designed to emulsify oils and grease with water and is the cheapest way. You use the vinegar to get a mild etch on the surface of the brass to enable the paint you are using to grab on. There is no need to dry the item after the dish washing exercise, just rinse it off and place it in the vinegar. If you have some heavy oxidation you can physically remove that with tooth paste and a soft cloth before the vinegar (or what ever etching solution you want to use, me I use phosphoric acid ). After the etching you need to make sure the item is completely dry before painting. You also need to get some of those gloves that Bob was talking about so you do not leave oil from your fingers on the surface, they are unbelievably handy, I have no idea how I did it before they came along. It is best to move from etching to priming or first coat fairly quickly so you can get the benefit of your work. I always try and do a complete covering for the first coat so that the paint forms a glove over the model and pulls down nice and tight and uniform. There after it does not seem to matter if I paint half the model or the lot. Just a side note to those gloves, if you are using compressed air to blow dry the model even though the air pressure is very low care should be exercised as to not let it blow directly on to your skin hence the gloves act as protection for you as well.   

[OldEastRR]

thanks for all the info. I think the vinegar was my missing step. We'll see.

[robert3985]

thanks for all the info. I think the vinegar was my missing step. We'll see.

You're going to "see" that vinegar isn't the answer.  Bestine is your answer.  Ah well....live and learn!   

[peteski]

You're going to "see" that vinegar isn't the answer.  Bestine is your answer.  Ah well....live and learn!   

From my own experience MEK should be at least as good at degreasing as heptane (Bestine).  While I have it, I personally do not use MEK for degreasing (too many nasty warnings about how dangerous it is).  Instead, I either use acetone on naphtha. Or 99% isopropyl alcohol.  From all the advice given I think that the one to use dish-washing detergent is very good. Then degrease it again using acetone or naphtha.

Since his paint is flaking off just from handling I suspect that there is more to it than just oily residue.  Maybe he is spraying the primer on too dry.

[robert3985]

From my own experience MEK should be at least as good at degreasing as heptane (Bestine).  While I have it, I personally do not use MEK for degreasing (too many nasty warnings about how dangerous it is).  Instead, I either use acetone on naphtha. Or 99% isopropyl alcohol.  From all the advice given I think that the one to use dish-washing detergent is very good. Then degrease it again using acetone or naphtha.

Since his paint is flaking off just from handling I suspect that there is more to it than just oily residue.  Maybe he is spraying the primer on too dry.

You could be right.  Somewhere along the line, he's not doing something, or doing something incorrectly.  The advantage of heptane over MEK is that heptane doesn't attack plastics.  I use it all the time for degreasing plastic assemblies before painting. It gets rid of any remnants of mold release too.  I wouldn't hesitate to use MEK to degrease a metal assembly (with proper venting), but not a composite brass/plastic assembly.

[OldEastRR]

My paint isn't flaking off like a rusted steel railing -- they are just tiny little chips on the body, sometimes along the edge of the brass, sometimes in the middle of the side. Maybe dust particles kept the paint from sticking there?

[robert3985]

My paint isn't flaking off like a rusted steel railing -- they are just tiny little chips on the body, sometimes along the edge of the brass, sometimes in the middle of the side. Maybe dust particles kept the paint from sticking there?

Dust won't make the paint "flake".  It's sounding more and more like you're getting atomized oil from your compressor coming out of your airbrush, which will do exactly what you're describing.  If you don't have an "oilless" compressor, and/or you don't have a robust filtration system on your air line, you're gonna get oil in your air, as well as condensed water, neither of which is going to be good for applying either acrylic or enamel/lacquer based paints.

[OldEastRR]

It's only on metal that the paint has problems, I've airbrushed dozens and dozens of plastic things and the paint sticks tight with no blemishes.

[havingfuntoo]

Clearly the paint is not keying to the surface in a uniform way, go on give the vinegar a run and see what happens. I am sure you will be surprised at how good the finish is when you prep the way the professionals do. Ok they don't use vinegar but it will enable you to carry out a similar process. 

If you feel unsure do a little test, take a piece of brass, prepare one end in the way I have suggested above, leave the middle of it as is and then prepare the other end using one of the versions described by the vinegar doubters.
Paint the whole thing using your normal methods.
Let it thoroughly dry, even bake it if you want.
Then do the scratch test and the bend test and watch what happens to the paint.

I look forward to your report.     

[robert3985]

Clearly the paint is not keying to the surface in a uniform way, go on give the vinegar a run and see what happens. I am sure you will be surprised at how good the finish is when you prep the way the professionals do. Ok they don't use vinegar but it will enable you to carry out a similar process. 

If you feel unsure do a little test, take a piece of brass, prepare one end in the way I have suggested above, leave the middle of it as is and then prepare the other end using one of the versions described by the vinegar doubters.
Paint the whole thing using your normal methods.
Let it thoroughly dry, even bake it if you want.
Then do the scratch test and the bend test and watch what happens to the paint.

I look forward to your report.     

Good idea.  There's no substitute for experimentation.   I'm not a vinegar "doubter", I am fully aware that etching a metal surface aids in paint adhesion...I'm just saying it's not necessary.  I've painted many many brass assemblies since starting out as an industrial model builder with Battelle Labs back in 1972, not just trains...and I've never vinegarized a single one of 'em (if I need "tooth" I use my grit blaster), and the paint always stuck as long as the air was filtered, the part degreased and I kept my fingers off the surfaces to be painted. 

I too am looking forward to results. 

[havingfuntoo]

No dispersions were being cast at you Bob, all is good, grit blasters do a similar surface preparation depending on the medium used, and how new it is will dictate to how aggressive the surface attack is. I find that I am not that controlled with the grit blaster and have ended up with some very unexpected results, not what I was wanting. To reiterate one of your early points Bob, it can't be emphasise enough the value of wearing gloves when handling models during the painting process.