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Stumped: How to Stop Oil-Based Enamel From Yellowing

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RocketRoll

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My Estes Saturn V is all painted and ready for the 50th anniversary of Apollo 11 next year. I used Testors gloss white spray enamel and Testors gloss black and silver (bottle enamel). The decals in the kit are incredible. I wanted to clear coat it with Krylon UV-Resistant matte acrylic, but...

After reading about yellowing issues on here and all over the internet, I'm stumped about how to prevent it. This is how I understand it:

1.) Enamel paint yellows over time. Many clear coats also yellow over time.
2.) Oil-based enamels (Testors) yellow faster if kept in dark areas (so I should expose the rocket to sunlight). Water-based enamels yellow faster when exposed to sunlight (N/A for me because I didn't use these).

What is stumping me:

1.) Does the anti-yellowing aspect of the "UV-Resistant" clear coat mean that the clear coat itself resists yellowing, or that it prevents the underlying white coat from yellowing? Or both?
2.) Let's say I don't clear coat the rocket at all. I keep it in a room with "slight-to-moderate" sunlight. No smoking, cleaning with ammonia, mostly climate-controlled, etc... how long before I start seeing some yellowing? What about the decals?

Winter is fast approaching here in New England and I'm running out of painting days if I do decide to clear coat it. And after all this effort, I don't want my brand-new Saturn V to turn a nice shade of yellow by the time I launch it next year...

Any help on this?
 

Micromeister

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RocketRoll:
Yes most of what you've posted is correct for the individual painted and products you listed. That said there is much that can be done to REMOVE the yellowing tendency from many of the offending products.

NUMBER ONE: Testors Paints: ALL TESTORS paints have a tendency to Hue slide over the life of the dried film surface. Whites, Beige, Tans and Yellows do in fact yellow, White yellows badly in months not years after application. These yellowing changers are caused by the fact Testors paint contain 0% UV additives. Testors Gloss Clear, Matte Clear and brush on clear coat also contain 0% UV additives. They will yellow under ANY UV light source indoor or out. Most True water slide decals are over-coated with NAZDAR Clear Silk-Screen Ink unfortunately it like testers clears contain 0% UV additive. That said Nazdar Clear yellow far slower then most any Rattle can clears.

As a matter of fact until the introduction of Krylon and now several other rattle can UV resistant clears. All old style Clears Krylon #1300, #1301 and others would yellow within a year of so.

One more thing about All Rattle Can Clear coats. As they contain no pigments there is no platelet cross-link overlap to lock everything into a single strong dry film. As such Rattle Can clear coats become extremely brittle shortly after complete curing. Bumping around in the trunk, a hard landing or two. and just handling will cause these coatings to crack, lift and take a little of whatever is below it as it flakes off. If decal or paint you'll have at least two layers of damage to deal with.

Now to help you out with your Saturn-V. OK you have it painted with Testers White, Black and silver. It's OK in 1970 I painted my first (and still flyable after 139 flights) Saturn-V with Testors White, One-Shot Black and Krylon Silver metallic. I didn't use Testers Clear or other clear coat over my decals. Instead I used a premium auto wax to protect my decaled surface. I did not know that premium auto-wax contained various amounts of UV protective components. You have much better options. Krylon UV Resistant Clears have been around now for more then a decade. When the material first appeared on the market I decided to do long term direct Sunlight yellowing tests on the Krylon 1305 gloss UV Clear, 1309 Matte UV Clear and 1306 workable Fixatif. All three have been year round exposed to southern sunshine since 2002. To date Not a single panel has shown the slightest tinge of yellow. Since then I've checked on my 20 some test pieces ever 3 months without any sign of yellow.

My strongest suggestion is to use Krylon matte UV clear over at least one solid wet coat of Krylon UV Gloss clear. This will prevent the Testors White from receiving much UV even outdoors all day.

If your Titanium white painted models ever do yellow they can be cleaned with 3m Finessit-II. This finishing material is not a wax or rubbing compound but an extremely fine surface refinishing material that will remove a small amount of paint along with any clear over-coating and bring the white surface back to nearly pristine. A couple examples below.
Hope these help a little.
 

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RocketRoll

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Thanks for the info - one question though. If oil-based enamels yellow due to lack of light (I have no idea how this works), then keeping it in sunlight would prevent it from yellowing, without the need for any clear coat, right?

Conversely, for paint that yellows when exposed to light, storing it in complete darkness would prevent yellowing, right?
 

neil_w

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My strongest suggestion is to use Krylon matte UV clear over at least one solid wet coat of Krylon UV Gloss clear. This will prevent the Testors White from receiving much UV even outdoors all day.
Ooh, now this is interesting. Why gloss before matte?

Also: you attached a picture of the Future bottle but didn't mention in your text. My Future-coated models have not yellowed. Can I assume that Future has UV-protectants?

I feel like, for gloss models, I'm going to stick with Future until proven otherwise. For matte I'll use the Krylon, although I'm still open to the idea of using additives to Future to dull the gloss a bit (I'd be happy with Satin usually; true matte is hard to keep clean). But I'm awaiting a proven way to do that effectively.
 

Micromeister

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Neil:
The use of clear is to completely seal the underlaying paint. Matte and/or Satin clear coats contain agents that allow migration of moisture and thinners. So seal the paint, UV clear coat it, apply decals, and UV matte clear to get the correct finish you're looking. Please note the Future or Nu-Finish can only be used over Gloss finishes as they are also High Gloss Finishes.
 

gpoehlein

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I have one other weird suggestion - You might try using a cheap airbrush to paint your model with liquid paints rather than rattlecans. The reason is that you can mix your paint before using it. And, a little trick I discovered years ago when painting Star Trek starship miniatures, is to mix in a drop of light blue paint into the white. This is the same trick as adding bluing to your whites in the laundry - a slight tint of blue actually makes the white look whiter. Try a test and look at white paint side by side with the same paint with a drop of light blue added. The pure white will look dingy and yellowed in comparison.
 

tomsteve

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Ooh, now this is interesting. Why gloss before matte?
to add to what was mentioned, applying coats of matte only can make whats underneath hazy looking. if looking for a matte finish and doing multiple coats, its best to make the first coats gloss then final coat matte.
 

DeltaVee

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Question for you, Micromeister....

I prefer the Rustoleum 2x brand of paint now... while it's not as run-forgiving as Krylon, I find the coverage far better and the surface of a good wet coat to be almost mirror smooth... at the cost of a somewhat slower drying.... but to ask an actional question:

Do you happen to know if the Krylon UV clearcoats will have any ill-effects on this paint (it's been more than a week since I applied the paint so it's quite dry.)
 

neil_w

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to add to what was mentioned, applying coats of matte only can make whats underneath hazy looking. if looking for a matte finish and doing multiple coats, its best to make the first coats gloss then final coat matte.
I experienced this hazing in a couple of spots on my IRIS-T, which was my first time ever applying a spray matte coat. In the future I'll do the gloss first than finish with matte. Learned something today!
 

Micromeister

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Question for you, Micromeister....

I prefer the Rustoleum 2x brand of paint now... while it's not as run-forgiving as Krylon, I find the coverage far better and the surface of a good wet coat to be almost mirror smooth... at the cost of a somewhat slower drying.... but to ask an actional question:

Do you happen to know if the Krylon UV clearcoats will have any ill-effects on this paint (it's been more than a week since I applied the paint so it's quite dry.)
I'm using the same Rustoleum 2X and other Rustoleum paint products now rather then Krylon (except Old formula) when I can find them:). Krylon UV clears have no ill effects on any Rustoleum product I've used. I do generally go with a light misting coat first over the new paint then a heave wet coat to seal.
 

Trident

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I started using the Krylon UV-Resistant Acrylic Clears about 6 years ago. Very pleased with my “long term tests” so far. No yellowing whatsoever.
I am also a fan of the Rustoleum 2X spray paints. Back about 20 years or so I used the Krylon Interior-Exterior spray paints. When Krylon changed to the Indoor-Outdoor formula, I quit using it. I guess I probably tried to paint using the technique I used with th3 old formula. The old formula is available mail-order. Krylon now calls it Acryli-Quik, it’s an acrylic lacquer. A user over on the other forum loves it, says it’s the same old paint, with a new name. The only problem is that you have to buy it by the case — 6 at a time. But many places offer free shipping on orders over $50. When my Rusto is gone, I’ll likely start out with black and white Acryli-Kwik initially, maybe add red, orange, and yellow if I can find someone to go halves and share it. I’m not really excited about spending $200 or more all at once on paint. :)

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