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SkyKing

On decals and clear coats and lacquers and enamels.

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I recently purchased some decals from a well-known decal designer who specializes in decals for sci-fi subjects. They are beautifully printed and in perfect register, and I would not hesitate to recommend them to anyone. However, the instructions for these decals contain this claim: "A known fact is that you can apply lacquer over enamel, but not enamel over lacquer."

This "known fact" was not known to me. In fact, it contradicts everything I have ever read or learned about lacquers and enamels in over 60 years of building models. The rule has ALWAYS been acrylic (water-based) over enamel over lacquer but not the reverse.

The instructions also state "Using an enamel clear coat such as Krylon or Rust-o-leum may cause damage to the decal." I've never known anyone who uses Krylon or Rust-o-leum on their models and wouldn't recommend them in the first place.

I questioned him about this and was told: "I've been making decals for over 17 years and early on those who used Krylon Enamel sprays found it ate their decals. They use it because it was cheap and it ruined their finish and they had to repaint their kits. The only cause we could determine was that it was the enamel. At that time I spoke to a friend of mine who did autobody work. He told me that the rule was lacquer could go over anything, but not enamel."

I've never heard of this "rule." I suggested to him that an auto body specialist was not necessarily the best person to ask about painting plastic models, and that his "known fact" may result in some damaged models and some unfortunate and unwarranted criticism of his decals when a novice modeler sprays a model which has been painted with enamels with an incompatible lacquer clear coat. I also suggested that he correct his "known fact," advise modelers to use paint products intended for plastic models, and eliminate any reference to DIY spray paints.

 

Comments, anyone?

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I paint enamel over lacquer on a regular basis. I prefer to use lacquer-based primer to prime bare plastic before painting, and make sure that it goes on a little wet so the solvents in the lacquer paint have time to etch the plastic a little bit to get a strong bond. I've also used clear lacquer as a barrier between color layers of enamel paint that I am brushing on, without any issues. The cheap clear-coats like Krylon and Rustoleum? The only reason I can think of that they might possibly damage decals is that they often contain xylol. Not sure if that's the cause, but that would be my guess. Xylol was one of the things we found when building experimental, composite aircraft, that had a slow evaporation time and was actually capable of softening cured polyester resin. Not many things can do that, so its pretty nasty stuff. Might be why he warns against that.

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I think there are two keys that haven't been mentioned that actually make BOTH of your rules "right": Cure time and "strength of application"; and both of these are interdependent on each other.

 

In my modeling experience, you can put anything over anything IF you let the previous coat completely cure (not just dry) AND apply the "next" coat LIGHTLY.

 

As modelers, we tend to be in a bit of a hurry. We don't let paint cure, we let it dry to the touch or to the point we can mask over it. This works well until we try to mix a harsher chemical over the previous paint level. It's not "cured", and thus it's susceptible to being eaten into by the new top coat. Allowing time to cure minimizes or eliminates this problem.

 

Also, most of us tend to be a bit heavy handed in paint application. After all, if we can apply one coat that covers, why waste our time misting on 2-3 coats? This is compounded even more when we use spray cans, where we have even less control on the amount being applied and we tend to put an even heavier coat on. THIS is where using commercial Krylon type clear coats can cause problems. Not only are they harsher in chemical make up than most hobby paints, but you're also spraying a much heavier application with them making it more likely it will react with the undercoat.

 

This heavier application is probably the real problem when it comes to your decals. Many low run commercial sheets are printed with low grade (or no) clear top coat. That makes whatever you spray over it likely to eat into the actual color coats and ruin them.

 

The "rules" you quoted are good ones, and will generally help you avoid problems. However, the guy you spoke to can also be correct. The best bet is not to "mix" paint types unless you have to. If you do, and you allow extra cure time and are careful in applying the next coat, you should be able to 'git 'er done.

 

GIL :smiley16:

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Gil's rule about putting the top coat on lightly is the key. Most of the solvents evaporate before they can do any harm. Rattle cans of any brand are renown for applying the paint a lot thicker than an airbrush, so that might have been the problem.

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