Paint storage question, Triarius?

Hi Don,

I think heat can be a factor, but also remember that many oil-based paints cure by oxidation.

In a paint jar where a quantity has been used, there is a higher volume of air (and therefore more oxygen) which may accellerate curing of the remaining contents. This may explain why you may have unused jars of the same age which are fine but partially used jars have gone off under the same storage conditions.

I have several unused/unopened jars of the old "square bottle" Testors enamels which I'm sure are pushing the 30-year mark, which appear to mix perfectly well when shaken.

Once you open the bottle, it will go bad, with just those symptoms. I know someone who squirts nitrogen in bottles when he closes them again (from a little spray can) but to me it isn't worth it.  I don't think temp has much to do with it, mostly having them open to air even for short periods.  Shelf life of unopened bottles is okay, but once opened both the Testors little rectangular bottles and the MM go bad pretty quick.  Personally I think it is the thinner vapor leaving the bottle more than the air getting in.

My experience is mixed it seems.
I've had bottles go bad that I've never opened and I have bottles that I use frequently that I use until they are gone and they never go bad. All MM enamels, as an example, I've had brand new unopened bottles of the RLM color line turn to rubber. But flat black, OD, interior green, and chrome silver get opened multiple times when I'm building and I usually end up throwing out the empty bottle before it dries up or turns to rubber.
I tend to believe it has more to do with the formula they had to use to get a particular color than it does whether or not it's been opened.

hkshooter

I have bottles that I use frequently that I use until they are gone and they never go bad.

That may be a key point. If you use them frequently, there isn't that long period  of storage while partially used. If you finish off a bottle within, say, 2 months, it's not the same as using half a bottle and leaving it to sit idle for 2 years before re-use.

hkshooter

But flat black, OD, interior green, and chrome silver get opened multiple times when I'm building and I usually end up throwing out the empty bottle before it dries up or turns to rubber.

Consider that these are frequently used colours for many applications and your LHS may have a quick turnover of these colours, meaning that they are always fresh when you buy them. This may not be so for some specialist colours which aren't often used. For example, we get Tamiya's enamel paint range here in Australia, but they;re not as popular as the acrylics. At one hobby shop, I have seen bottles of some colours literally dried out on the paint rack, and other more frequently used colours (like gloss white, gloss red) full to the brim. (I will note that Tamiya's enamels will dry out in the jar in about 4-5 years regardless of whether they've been opened or not.)

Interesting points, some I had not considered.
Regardless of what causes the paint to go rubbery I'll keep using them as long as they sell them. To me, acrylics are far to finicky to deal with, extremely delicate once applied, and will lift at the drop of a hat wtih masking tape. In comparason, enamels are nearly fool proof though I had a recent FUBAR with Humbrol not to long ago. I like the color range much better also.

I'd agree with the basics of your statement hk but I prefer to use acrylics because of the smell/fume.

As for the paint drying out issue, I've never had a tin (humbrol/revell) dry out that was fully sealed. I also have some unused (have been unopened) little square bottles of testors enamel that would be 8 years that are fine. I also have used round testor enamel bottles that are 15+ years old.

On the flip side acyrilcs thinner does dry out in the bottle quick (from 6mth to 2-3 years from opening) but does mic quite well again with a generic thinner

Andrew