Say what you want about Humbrol ...

Latex caulk is particularly susceptible to some solvents found in acrylic paints. It also contains plasticizers that migrate very well, and may do strange things to the paint. The best modern secondary sealer, if you feel the need, is PARAFILM M. It was and is made for just this purpose. Stretch out a bit and wrap it around the top of the bottle, overlapping the cap/lid and the bottle itself.

Oooooh, I like the Parafilm M suggestion. I can get tons of that stuff! Thanks.

Thanks to life events, I have only recently returned to scale modeling. Now, I'm no chemist and don;t claim to be. I know doodly about stuff like plasticisers and solvent migration and other sundry "painty " things.

But, in going through my old paints, I found many bottles of enamel that had been stored upside down, per my usual habit, and are still useable after years. I can't say the same about the acrylics - some of them are un-usable. The results varied and I wish I had taken better stock as to what was good and what wasn't. But on the whole, the majority of them still had use left in them.

The only thing I can think of is that the enamel paint at the edge of the rim dried out enough to make a seal and prevent further degradation... which is what was found once the lid was off and in the end, is good enough for me. It is indeed important to keep the threads and sealing surfaces clean, as you sggest, and I am fussy about that. Its one reason why I decant with pipettes, so as not to get paint all over these surfaces.

I know that wine corks are not the same as paint bottle seals, and it wasn't intended to be taken literally (I have noticed that modelers interpret things quite literally, so I will be a little more careful). The anlaogy was used as one that illustrates the point and "sticks" well in the mind.

Thanks for your comments though. I wont use the latex caulk and will stick to good old petroleum jelly.

At least the old formulations of Humbrol were some of the best paint I have ever encountered, IMHprofessionalO. Don't know about the newer stuff—but it's the only enamel I have or am willing to buy.

I agree with everything dahut said except inverting the container and latex caulk. The materials used to seal quality paint will not "dry out" like a wine cork. They should be kept absolutely  clean, however.

Latex caulk is particularly susceptible to some solvents found in acrylic paints. It also contains plasticizers that migrate very well, and may do strange things to the paint. The best modern secondary sealer, if you feel the need, is PARAFILM M. It was and is made for just this purpose. Stretch out a bit and wrap it around the top of the bottle, overlapping the cap/lid and the bottle itself.

Paints in plastic bottles, even acrylics, are likely to have a shorter shelf life than those kept in metal or glass. All plastics contain plasticizers the will migrate (slowly) into the paint over time, changing its properties, usually for the worse. Also, the more volatile solvents in the acrylics will equally slowly migrate into and through the plastic. 

I always liked the Humbrol tins, so I guess that makes me an oddball. They were a little cranky to open, as they required some sort of lid key. But, since I decant with a pipette anyway into a mixing vessel, it wasnt too bad. 

When I first started all this modeling business, paint was sealed after use to keep air out. This was especially so for specific colors - unless you were a fast builder, your bottle of a particular color could sit for a long time.

Two ways of sealing them were recommended.

1. Dip the lids of the jars in molten wax. This allowed the hot wax to work down to the lid/bottle join and seal it

NOTE: Using the dripping wax from a candle also worked and was easier, by far..

2. Work a dab of peteroleum jelly around the lid, sealing it off. You have to be careful to keep it away from the opening, though, and you have to inspect it occasionally.

Today, I think a little Latex caulking might do the trick and it was always prudent to stand your bottles upside down. This has a silimar effect as wine keeping a sealing cork swollen.

Daywalker, if you open a bottle of MM flat black, you've got about 5 minutes

Nah you're right about the bench life... it's only about a month or two. Which sucks because I like that paint 

RadMax nailed it with his statement about the containers being airtight. Keep the air and in some cases the light out the paint should last a very long time. Finding the right mix of cap and container is hard. Tins are probably the most durable, as long as the mating surfaces between the lid and tinlet are clean before resealing. I'm guilty of resealing my paint jars without wiping down the rim and threads before screwing on the cap that has paint clinging to the inside threads. So each time I unscrew the cap off, I get the crumbs all over my bench.

I have a bunch of really old Tamiya paints that still look great in the jar.  As for the MM, what is the shelf life of flat black?  I could never seem to get that one to last more than a month or two after I opened it.

I bought two tins of Humbrol in about 1983-84.

They're still good!!

I don't use Humbrol much anymore because of lack of availability locally, but I used it extensively in the past.  I have some tins that must be 10 years old & it's still good.

Regards,  Rick