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Paint rash on older diecast models

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Zondaracer

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Paint rash on older diecast models

PostMon Oct 12, 2015 5:20 am

I can't remember when the last time was when I dusted my models, but this weekend I did. And I found all my older models having paint rash. That's about 80% of my 50 cars collection. Sometimes it's just dullness but mostly pitting and small blisters. All marques are effected, from cheap Chinese stuff, Sunstar, ERTL, Welly, Kyosho but Autoart as well. I found rash on the bonnet of my Bond Lotus submarine (and no I didn't test it in the bathtub). It seems Maisto models are the best surviving. All models are in a cabinet, smokefree, sunfree. I'll hope I can polish most of the rash away, but I am not happy.Any idea what can be the cause of this? More people having problems with older cars?
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[KRAFTIG]

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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 9:21 am

I believe this is one of perils of collecting, everyone is susceptible. Like you, I have all of my models in sealed cabinet, smoke free, and window blind don't 99% of time. Each the best like CMC with their claim of the "best" Australian diecast metal are not immune. I find Kyosho to the worst offender, main reason I replacement some their models with AUTOart.
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StratosWRC

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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 10:24 am

This is what Exoto told me:

Dear Wes, this is a normal technical phenomenon in die-cast manufacturing. It does not happen always, but sometimes. We call it the PATINA of die-cast... Zinc being a porous material, even with all technical precautions taken, will sometimes release tiny bubbles of air that can potentially show on the surface of the paint.
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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 12:25 pm

PATINA Definition

Patina (/ˈpætɨnə/ or /pəˈtiːnə/) is a thin layer that variously forms on the surface of stone; on copper, bronze and similar metals (tarnish produced by oxidation or other chemical processes);[1] on wooden furniture (sheen produced by age, wear, and polishing); or any such acquired change of a surface through age and exposure. Patinas can provide a protective covering to material, patina is a coating of various chemical compounds such as oxides, carbonates, sulfides, or sulfates formed on the surface during exposure to atmospheric elements (oxygen, rain, acid rain, carbon dioxide, sulfur-bearing compounds), a common example of which is rust which forms on iron or steel when exposed to oxygen. Patina also refers to accumulated changes in surface texture and colour that result from normal use of an object such as a coin or a piece of furniture over time.[2]

Archaeologists also use the term "patina" to refer to a corticated layer that develops over time that is due to a range of complex factors on flint tools and ancient stone monuments.[1] This has led stone tool analysts in recent times to generally prefer the term "cortification" as a better term to describe the process than "patination".[3]
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StratosWRC

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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 12:54 pm

I guess it's a natural process.
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Zondaracer

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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 1:50 pm

On Facebook it was mentioned that the dust itself may be the cause. Could that be? Anyway I will polish the models and use a good quality wax for protection. I hope the models will not further degrade this way. But ABS models from Autoart may be my future.
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StratosWRC

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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 4:08 pm

Nah can't be dust
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[KRAFTIG]

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Re: Paint rash on older diecast models

PostMon Oct 12, 2015 11:07 pm

StratosWRC wrote:Nah can't be dust


Agreed.
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Re: Paint rash on older diecast models

PostThu Oct 22, 2015 10:03 am

I've had some NOS models that are 10-15 year old from Minichamps, Kyosho, and Exoto out of the box for the first time and found rash unfortunately so it can't be the dust. Old UTs on the other hand seem to be holding up quite well. I've also found some rash on a few early AutoArts as well.
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Re: Paint rash on older diecast models

PostFri Feb 05, 2016 11:28 am

Scary stuff here, I'm having visions of paint rash on my precious 787b. :shock:

Am I to assume that resin models will be immune to this issue?
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[KRAFTIG]

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Re: Paint rash on older diecast models

PostFri Feb 05, 2016 1:24 pm

groupCin1.43 wrote:Scary stuff here, I'm having visions of paint rash on my precious 787b. :shock:

Am I to assume that resin models will be immune to this issue?


Yes, but how will they age? Uncharted waters sort of speak.
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Re: Paint rash on older diecast models

PostWed Apr 26, 2017 8:07 pm

Tried the method described here:

http://www.diecastxchange.com/forum1/topic/165969-removing-paint-rash-from-diecast-models/

Tried it on one of the least rare/valuable (but not the least loved) models first. Maybe my experience helps somebody.

Lessons learned:

* Paint thickness on my model cars (AutoArts, Minichamps, Kyosho) is around 80 - 130 microns on average. It is good idea to measure thickness on used models because they may have had polishing before. And remember that paint on edges may be much less thick so you should avoid rubbing edges. I used paint thickness meter that I bought for 90$ and use when purchasing used 1:1 vehicles.

* Micromesh grit's crystal size is as follows (according to this http://www.sisweb.co.../conversion.htm + common sense)
4000 = 5 microns
6000 = 4 microns
8000 = 3 microns
12000 = 1-2 microns
but it sands off less, because full size of the crystals is not used, they just stick out to some degree. I was able to rub the same spot like 50 - 100 times with 6000 grit without going through paint.

* While 12000 grit may work on some light scratches and patches, for paint rash removal to work in reasonable amount of time, you have to use 6000 or even 4000 grit, as scary as it may sound.

* For me the best way was to sand off the paint rash with 6000 paper (this took most of the time, because you must be very careful, but also rub deep enough to get paintrash out), then a little bit rubbing with 8000 to smoothen out, then a lot of polishing with 12000 and 2 goes with each of 3 Tamya polishing compounds.

* If done accurately, it may take a lot of time.

* If paint rash bubbles are big, it may be a good idea just to smoothen them out to some degree instead of trying to sand them off completely. This will still look much better when all steps are complete.

* It will not look as good as new, but it will be much much better than with rash.

Tested on Autoart, non metallic paint. So these tips may not apply or may differ on something else.
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Re: Paint rash on older diecast models

PostWed Apr 26, 2017 9:53 pm

Thanks for the info. Me personally I'd be too scared to attempt it.
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Re: Paint rash on older diecast models

PostFri May 26, 2017 11:17 am

I used this method on several older Autoart and a CMC that had paint rash with great success. After the models were just like new or better. Takes a lot of time to do things right and make sure you don't damage anything but in the end it is really worth it.
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Re: Paint rash on older diecast models

PostTue May 30, 2017 11:04 pm

Zinc is not a particularly stable 'metal' when it comes to corrosion resistance. It is very good as a sacrificial coating when plated to steel via electrolysis. However as a solid product I think it is quite poor.

Diecast metal (primarily zinc) is a mixture of several metals; aluminum, lead, probably others. Dissimilar metals almost always create an opportunity for galvanic corrosion; which is apparently what happens with a solid zinc casting.

The fast casting process does not help: molecules are not really well aligned and the granular structure is relatively poor. If you cut a relatively thick part of a zinc casting it is likely you'll see air pockets.

Why such a crappy metal, you might ask. From what I've read, high zinc content alloy doesn't need high-temperature to cast and flows relatively well to fill the correct shape. (The quality of the negative mold is a different story). It is also cheap.

Also, I don't think very many manufacturers use a sufficient, if any at all, primer layer. I can't say for higher-end models, but whenever I've sanded through a panel there is no primer layer. I think a proper primer would help slow (but not stop) the galvanic process manifesting itself on the surface.

Of course for most models I cannot say. But for all of my models that have paint chips or that I've sanded, I can only think of one that has a visible, original priming layer. And it is a 1:64.

This doesn't provide any answer to your query. However if the cause is galvanic corrosion (which is likely), it cannot be stopped. The only fix stripping the paint, properly prepping the metal and using a primer formulated for this base, then a topcoat. That however, isn't practical and only slows the process instead of permanently fixing it. The best is probably to keep an eye on the finish and correct it before it gets worse.
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Resin is of course immune to corrosion. But being a plastic product it has its own problems. Sagging, and/or inability to maintain original shape, especially under prolonged UV exposure, after many years are the biggest problems from what I have read. Quality resin models are very thick not only to have the heavier feel of a metal model, but also to resist warping.

And of course most resin products don't have opening parts. I imagine the likely causes are 1) it is a very soft material and would not take well to many mechanical metal hinge attachments but 2) would probably have very high rate of failing a QC inspection - holes are that much easier to strip out.

I imagine it could be done on a large scale if the hinge systems are metal and bonded to the resin with adhesive rather that a mechanical system. Jigs would help ensure consistent hinge placement and, consistent panel gaps. However panels would need to be so thick (in order to resist warping) that opening them may not reveal much!

Doors could be done with the interior panel as part of the door casting in order to achieve thickness, however such a two-sided mold is far more expensive than one-sided. Larger panel openings (think Zonda, F40) are probably achievable with room to thicken and strengthen the resin. However a standard hood and trunk, given their needed thin construction, are probably not doable.
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