A quick (chemistry) lesson on how varnishes cure.

 

Stage 1:   The thinners, for example mineral spirits, evaporate.  This is known as "flashing off."  This happens within a few minutes.

 

Stage 2:  There is a chemical reaction.  Oxygen in the air is combined and the resins form long polymers (chains) that make the varnish into a solid.  This starts over a few hours, but takes about 3 weeks to complete.  This is also why a half-full can of varnish (the other half having some oxygen in it) will gel over, the reaction has started.  And why just adding more paint thinner does not un-gel it.  It's a thinner,  not a solvent.  The reaction has already started.

 

Last week, I was doing some on-site varnishing, so I put some varnish in a plastic jar.  I just left what was unused in there.  A week later, that little bit of varnish grabbed all the oxygen in the top of the can and collapsed the sides.  See photo

 

Bonus lesson:

A similar thing happens with oil finishes like boiled linseed oil.  Except there is not usually any thinner to flash off.  The general rule is that a chemical reaction's speed doubles for every 18F or 10C degree increase.  This is why these finishes cure slower in a cold environment and faster in a warmer one.   The oil polymerization is exothermic, meaning it gives off heat.  So if you put an oil-soaked rag in a wad, The following happens:

    - The finish cures and puts off heat, trapped inside the wad

    - The heat increases the reaction, producing more heat

    - Repeat until the ignition point of the oil is reached and a fire occurs

 

 

 

Thanks for the reminder, Keith. Hopefully everyone will read this... it may prevent a disaster!

I have discovered that putting poly or Danish oil in wine bags and squeezing out the O2 will help extend the life.

Also lots of other stopgap but in my opinion almost futile ways to extend can life . Some are to replace O2 with an inert gas (Bloxygen), exhale into the can (more CO2 in breath than O2).

Thanks for the explanation of the process.

I have a small squeeze bottle of mineral spirits I keep in the kitchen.  Good for removing most label adhesives.  Don't know which plastic it is, no markings on it, but the the mineral spirits will disappear from it.  Permeation I believe.  I switched it to a bottle that had sanitizer in it labeled PETE and it does not disappear out of it.  Something to think about when storing stuff in plastic jars.    

3 hours ago, Gerald said:

I have discovered that putting poly or Danish oil in wine bags and squeezing out the O2 will help extend the life.

Also lots of other stopgap but in my opinion almost futile ways to extend can life . Some are to replace O2 with an inert gas (Bloxygen), exhale into the can (more CO2 in breath than O2).

Thanks for the explanation of the process.

I have a couple of StopLoss bags.  One I have had Waterlox in for about 5 years without problems.  In my experience, Waterlox gels pretty quickly if you don't do something to keep it away from oxygen.    I have a friend who worked for Proctor&Gamble and one of his jobs was to develop a lining for one of their products (maybe shampoo) that would be an impermeable barrier on the inside of a shampoo bottle to keep the fragrance and moisture in.  It seems that many plastics are not impermeable.  It looks like Jif peanut butter jars don't let a lot of air thru either.

 

I have put packing bags inside partial cans of varnish to fill up the empty space.  I tried propane, but it did not seem to work as well as Bloxygen might.

I’ve experimented with various “air dusters”. So far nothing is working. If anything they are accelerating the gelling process. Guess I’m going to have to bite the bullet and invest in Bloxygen 

The wine bags are very cheap and are very easy to use product from them . In Pharmacy school they taught us that all plastics are permeable to air and therefore prescription storage packages we receive drugs in are made of special plastics. Once the original package is opened the drug is exposed to air and moisture and therefore the expiration of the drug changes. Just don't ask me what kind of plastics

28 minutes ago, Gerald said:

The wine bags are very cheap and are very easy to use

Gerald, where do you get these bags?

I'm using Stoploss bags right now, and they do work well, although I find getting the finish into them from the can to be....challenging. But over the years I've tried all sorts of gas toppings like propane and homemade CO2 (mix vinegar and baking soda to produce it) and none worked. They may have extended the gel covering, but didn't stop it. (Disclaimer, I never tried Bloxygen). If you remember those plastic freezer bags that had a hand pump to suck the air out, I tried buying some of them and cut the valve out. Then I taped that valve onto the finish can and punched holes under it, using the hand pump to suck the air out of the can. In one instance I actually caused the can to collapse. Interesting, but that didn't work either.  I have read of nothing but success getting the air out of the can by dropping in something to raise the level of the finish to the top...usually marbles but anything that doesn't dissolve should work, like clean gravel or ball bearings. For me, the Stoploss bags are working well enough I won't be experimenting anymore.

20 hours ago, kmealy said:

This is also why a half-full can of varnish (the other half having some oxygen in it) will gel over, the reaction has started.  And why just adding more paint thinner does not un-gel it.  It's a thinner,  not a solvent.  The reaction has already started.

 

So if the partial can of finish has not begun to skin over can we feel confident that the finish is still usable no matter the age?

12 hours ago, lew said:

Gerald, where do you get these bags?

On Amazon look for alcohol pouches for liquor. There are several different sizes.

this is what I got but there are many choices: alcohol pouches

 

 

16 minutes ago, Gerald said:

On Amazon look for alcohol pouches for liquor. There are several different sizes.

this is what I got but there are many choices: alcohol pouches

 

 

Thanks!!!