Pigment leaching from plastic into paint

[Jakko]

I think I’ve mentioned this phenomenon before on here in some thread once, but it just happened to me again, so I thought I’d post about it.

Earlier today I sprayed the interior of my Sherman Crab with white Vallejo Surface Primer from a spraying can. As I was doing this, I noticed the Academy parts on the model turning a distinct greenish hue:



Notice the difference in colour between the paint on (on the one side) the outer walls, the bolt strip at the front and a few other bits, and (on the other) the transmission, floor, seats etc. and also the white sheet of paper underneath. The paint that got onto the Asuka parts of the outside bits is noticeably whiter than that on the Academy parts for the interior.

Or here:



The little box is from Italeri, the rest is Asuka (and white plastic card) underneath the paint.

The only explanation I have for this is that the paint draws pigment out of the plastic underneath, but only for some types or brands. I first saw this happen on a Tamiya kit I had extensively converted, and all the Tamiya parts turned green with a coat of primer (not the same type as here) while the rest was white as they should be. Additional coats didn’t do much to resolve the problem, either.

Since in this case, these bits will all get a dark wash, a white drybrushing and weathering over them, I don’t think it will be a major problem, but it’s definitely something to be aware of if you intend to have an all-white model, if you ask me.

 

[Jim R]

Hi Jakko
How strange. I have never heard of it before or experienced it. I imagine that it is only really obvious, as in your case, if you are using a white or very light primer and what you are painting consists of parts from different kits or includes scratch built parts. The difference is certainly noticeable in your photos. I don't know what plastic is coloured with but maybe different manufacturers use plastic coloured with different colouring.
Jim

 

[Jakko]

It has only happened to me once before, or at least when it mattered, namely that Tamiya kit I mentioned. It was a Canadian M113 TUA in UN white, and all the dark green Tamiya parts turned pale green while the non-Tamiya bits (Verlinden resin turret, black Elite Models tracks, scratchbuilt roof, etc.) were clearly white. I think it took five coats of primer then to get it to look acceptably white. Maybe, in retrospect, I should have airbrushed the model white over the greenish primer

 

[Tim Marlow]

As I’m sure you’ve worked out, it appears the solvent in the primer is mobilising the pigment in the plastic. I’ve never had it happen, but think the only way to stop this is to seal the surface with a coat of a paint that uses a different solvent, such as Tamiya, or perhaps a thin coat of gloss varnish.
Think of it like a water stain on an white emulsion painted ceiling......you stop it coming through the fresh decoration with oil based undercoat paint. This seals the surface. You can’t simply re-coat with fresh water based paint because the water in the next coat of emulsion draws the stain through as it evaporates.

 

[Deleted member 7181]

It would be interesting to see the effects of using a different primer, preferably airbrushed.

As I'm sure we all know, white is a funny colour to deal with. You can paint something and it looks perfectly white, but as it dries it becomes quite transparent and the underlying colour can "work its way through" some time after painting so more coats are needed than you initially thought. However, you say that extra coats don't really help so it may be something entirely different going on here.

The spray cans are a likely cause. As well as the solvents to thin the paint, rattle cans generally include a portion of much "hotter" solvents to stop the nozzle from clogging in use. These usually evaporate really quickly and it's the reason most cans recommend a spraying distance of 20-30CM, as that will give these solvents time to disappear before hitting the surface. Since we tend to spray much closer in model work, maybe it's these extra hot solvents that are causing the issue.

I'm only musing here as I've never used spray cans on a model kit with different coloured plastics, but it's all that comes into my head right now.

 

[Jakko]

It certainly sounds plausible. I was spraying from closer than 20 or 30 cm much of the time. Maybe I should take some of the Academy sprues and test if distances makes a difference. Unfortunately those still have plenty of parts on them, though

 

[stillp]

As well as the solvents to thin the paint, rattle cans generally include a portion of much "hotter" solvents to stop the nozzle from clogging in use. These usually evaporate really quickly

Ah, so that's why people sometimes suggest decanting the paint and leaving it to "outgas" before using it in an airbrush. I've never been happy with the idea that dissolved gas would stay dissolved during the loss of pressure as it's sprayed from the can, but then evaporate if left in a container, so perhaps it's not gas but those hotter solvents evaporating.
Pete

 

[papa 695]

I would just use a grey primer then paint it white after, it looks like the white primer itself is becoming translucent.

 

[Deleted member 7181]

Probably Pete. Outgas or offgas are terms we use in the trade for letting the solvents evaporate, so it may come from that.

That was certainly my first thought Ian, and something I see regularly at work with both paint and rattle can primer. If I'm priming a multi coloured substrate I'll give it a coat of grey first to unify the colour, then white over that so that when the opacity reduces upon drying it's less noticeable.

Jakko mentioned that subsequent coats of white didn't do much to help though, which is why I was trying to think of something else that may be causing this.

 

[papa 695]

Probably Pete. Outgas or offgas are terms we use in the trade for letting the solvents evaporate, so it may come from that.

That was certainly my first thought Ian, and something I see regularly at work with both paint and rattle can primer. If I'm priming a multi coloured substrate I'll give it a coat of grey first to unify the colour, then white over that so that when the opacity reduces upon drying it's less noticeable.

Jakko mentioned that subsequent coats of white didn't do much to help though, which is why I was trying to think of something else that may be causing this.

Yes but Jakko used white primer again instead of white paint which has more pigment. and the primer could have been the problem all the time. That's why I would use the grey primer first.

 

[Deleted member 7181]

Ah, I missed that point Ian, sorry.

 

[Jakko]

Yes but Jakko used white primer again instead of white paint which has more pigment.

I tend to use white primer if I want something to be white because it covers better, so I always kind of expected that to have more pigment than normal white paint