How to get my nitrocellulose paintcoat shiny and prevent clouding?

Question

I am working on a prop helmet for LARP. It's made from foam and by now I have spent almost one and a half rattle cans of shiny black, high gloss nitrocellulose-based spraypaint on it in about 6 or 7 layers to attempt to remove the dreaded paint blush. However... it always turns a dull gray as the paint blushes/hazes/mists.

Because the helmet is made from high-density EVA foam and contact cement, I can't use a heat gun on the lacquer to try and dry it faster, and nitrocellulose lacquers are highly flammable anyway. It is sealed with Ponal Express, then primed with 2 layers of an automotive filler-primer and has accumulated quite some black paint layers by now. Nitrocellulose lacquer is not known to react kindly to other paint types getting slapped over it, with results from not adhering at all over bubbling to even noxious gas. As such, a different paint is not in the cards, as stripping the paint isn't possible without destroying the prop.

Is there a secret I miss during application of nitrocellulose lacquer to prevent the blush, or something I can make to get the dull surface to shine black? Test pieces were blush free and the blush itself isn't everywhere, so it must be a technique error.

I do NOT need a brand suggestion, the problem seems to be in some other factor like the application technique or the circumstances as I am getting some glossy bits at the edge and then all hazed paint.

Answer 1

What you're seeing with that nitrocellulose lacquer (equivalent to nitrate airplane dope, guitar lacquer or old style fingernail polish) is blush.

Blush is caused by the rapid cooling of freshly applied solvent based coatings generating condensation, which produces a matte surface rather than the intended high gloss. The solution is humidity control.

Either apply your lacquer when the local humidity is quite low (I've had good results with spray enamel at 20%, but lower would be better for a coating that contains ether or acetone, like your nitrocellulose lacquer), or take direct control of your local humidity by painting in an aggressively dehumidified room (while wearing a respirator with a suitable filter cartridge for the solvents you're using -- breathing concentrate ether and acetone is decidedly bad). Gentle heating while drying will do no harm as long as the air is very dry, but likely isn't needed if the air is dry enough to avoid blush in the first place.

Once you have humidity under control, you can wet sand the existing coats (600 or finer wet/dry sanding paper and water), dry thoroughly, and then apply at least two additional coats of your lacquer, observing the "fat over lean" rule -- that is, either use less reducer in the first coat and increase it with each subsequent coat, or if you're using cans, spray minimal amounts and then work up to heavier coats (observing recoating recommendations the can, often "recoat within 20 minutes or wait 24 hours" or similar).

Answer 2

Getting EVA foam to shine is a heck of a process. I have tried working with automotive paints in the past, and while those spray primers are appealingly easy to use, I have never been able to get a smooth, glossy finish when using them. Part of the problem is that you need a ton of layers of the primer in order to create a smooth surface, but there's also the matter of most automotive primers not being shiny enough.

Instead, I would recommend using a PVA-based primer. Some people use white Elmer's glue (the stuff for wood, not the washable stuff for little kids), but it isn't as flexible as some options. If the helmet isn't squishable, due to internal structure or something like that, then that isn't a concern, but one of the main draws of EVA foam is that it does flex. So, if you need it to flex, that might not be a great option.¹

I prefer to use Flexbond, which is a primer designed for theatrical backdrops. It can't be sprayed, but it is largely self-leveling, especially if watered down the appropriate amount. ² I've found that Flexbond works really well for giving a high-gloss shine.

If the high-gloss spray paint works over the new primer, great! If not, I recommend finding a good glossy acrylic. (There's a line of paints from Plaid specifically aimed at cosplayers that are gorgeous, albeit expensive, and they're very prone to sticking to each other, but we'll solve that in a second.) You'll also want to add shading along the contours of the piece-- highlights at the edges and shadows in the depths. This step is huge, and goes really far towards tricking the eye into believing that an item is shiny and metallic.

Finally, get a good high-gloss top coat. This is the only part where I prefer something sprayable over something brushable (but that's just because my airbrush is currently out of commission, so I can't use it for the acrylics). This serves two functions. The first is to protect your piece from dings and scratches, and also from sticking to other painted pieces. ³ The second is that, well, it adds a bit of extra shine!

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¹ Spray paints also don't flex, and you'll probably wind up with cracking over time if the helmet does get bent or squished in any way. Acrylic paints are much less likely to crack like that, as long as they're decent quality.

² I like to make mine the thickness of those probiotic drinkable yogurt shots-- just a little bit runnier than it is out of the bottle. Undiluted, you can get clumpy drips along the contours of the piece.

³ Seriously, this is not a corner to cut. I have made this mistake so many times, and wound up in tears because my paint job wound up ruined.

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I also want to offer a disclaimer that I'm not sure that any of these techniques would hold up as paint jobs for actual LARP combat, but given that the helmet is identified as being a "prop", and given the techniques and materials that have already been tried, I suspect that the goal isn't for this helmet to be taking blows.

Answer 3

Automotive primers require surface sanded to smooth satin, surface that's generally stiff like a steel. Similarly nitrocellulose paints call for a smooth surface that does not change dimensions easily.

Foam is not it. No way around it, foam does not have surface parameters of steel, aluminum or fiberglass, or even polished wood. If you can coat it with one of those, it should've been your building material from the start.

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What you need to do, is to use primer and paint designed for the surface you have.

I got good dark gloss using gunmetal FlexiPaint. I sealed my EVA with a heatgun (no need to do this if you didn't use abrasive materials on it), primed with FlexBond, and painted with Flexi Paint - Gun Metal, locally mixed with small amounts of acrylic silver or acrylic brown (for rusty parts). This paint is thick and has high surface tension, so it tends to even out on its own. It is advertised as not sticking to other stuff, but in my experience sealer is still needed.

My friend was using airbrush Alclad II primer and high shine paint on foam LARP weapons. In my opinion shine was much better, but because of that battle damage, not the designed one but one from the actual LARP fights, seemed to be more visible.

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Disclaimer: I am not affiliated with these brands. I linked shop only because manufacturer's page does not present colors individually and that shop is authorized reseller that does, and I bought from them without issues. Still, I can't vouch for them.