Paints going funky

[Arkhanist]

17 hours ago, Valkyrion said:

Cool, so you just kinda press down on it. I was expecting like, I dunno, you sit it in there and come back five minutes later to a mixed product. 

Ha! It's 3000rpm! I should read things properly. That goes someway to explaining it!  That's like your washing machine on steroids. 

Yup, very much like holding your pot against a tiny angry washing machine!

17 hours ago, Valkyrion said:

And this helps with the blobs? Or really for the paints that have separated?

Both, basically. Adding an agitator/mixing ball or two will help speed up mixing, especially for awkward paints (e.g. that white crap at the bottom of Contrast pots), I add them as standard to basically every new paint - I have stainless steel balls. Stuff that's lumpy or very thick will benefit from also adding a little medium or airbrush thinner as well - this is more effective than water at breaking up clumps, in combination with the mixing. Paint that's gone completely solid is too far gone though.

The way the vortex mixer works is it gets the paint swirling into a vortex, i.e. a whirlpool inside the pot. It takes a few seconds for it to get the paint up to speed, but then you can hold it on there as much as is needed to mix up the paint (or do it several times). For recently used paints, 5-10 seconds will do the job. For really crusty not mixed in years paint, such as old GW metallics, it may need up to a minute or so, plus thinner and a mixing ball. It does vibrate your hand somewhat (though not painfully), so you can do it in batches, i.e. buzz it for 20 seconds, then see if it needs more, and do it at a different angle. Also hold the pot firmly in your whole hand; it it possible to just press down the paint pot into the rubber top of the vortex mixer with a single finger, but more risk of it going flying! Also it is obvious, but do make sure the lid is on solidly *before* mixing...

I dug out a previous before and after of using my vortex mixer + steel ball in each (for about 30 seconds each IIRC)

Edited August 9, 2022 by Arkhanist

[Emperor Ming]

The little bits of crap that appear in the citadel whites drive me bananas

Even paint agitator balls don't seem to get rid of them

[Sothalor]

For some DIY paint pot agitators, I snip leftover sprues into little chunks and just chuck them into paint pots/bottles.

[Valkyrion]

For my own edification - what actually is it that causes paints to either go lumpy or separate in the pot? 
 

[Arkhanist]

I'll give it a go!

Acrylic paint is made up of 3 basic components.

1) The pigment is what gives it colour. Organic or mineral material is ground up into very, very fine powder. Different colours of pigment are mixed together to get the desired paint colour. (weathering pigments are basically this on its own). Incidentally, this is also the expensive bit, which is why modelling paint - with its greater quantity of finer-ground pigment - costs most than basic craft paint, and why it covers better and dries smoother.

2) the binder is a mix of acrylic polymers, different paint makers use different mixes, but they all have largely the same effect

3) the solvent is water

When all 3 are well mixed together... you have paint. However, the pigment is only held in suspension in the water. So pigment-heavy water tends to settle to the bottom (being heavier), and the binder-heavy dispersion rests on the top (the milky liquid). This is not really a problem, as strong mixing will combine them all together again evenly.

Lumps are caused by larger clumps of pigment powder sticking together instead of being evenly dispersed; this is a particular issue with titanium dioxide, a very large molecule, commonly used for pure white (or lightening other colours), which can be a problem to break up again - they tend to resist mixing.

As the water evaporates, the binder bonds to itself, the surface it's on and the pigment dispersed with it, forming a waterproof acrylic film. I.e. dry paint!

 

In addition to the above, there are also additives that can be used to alter paint's properties. Acrylic dries pretty glossy naturally, so a matting agent (often a talc-like substance) is added to the suspension to make it dry more matte. This is the white chalky substance you sometimes see at the bottom of contrast or washes.

Alcohol and glycols are both better solvents than water, i.e. can carry more pigment per volume, so either or both can be added to e.g. improve pigment density - i.e. better paint coverage (tamiya does this). However, GW I believe does not in order to keep their paint as nontoxic as possible. Either/both of these are usually the primary active ingredients of acrylic paint thinner, such as vallejo airbrush thinner, and help get everything back in an evenly balanced suspension more aggressively than water. Evenly mixed thin non-lumpy paint is an absolute must for airbrushing, hence why they also work nicely to even out brush paint also, though in smaller quantities. (scale 75 and tamiya both react poorly to general thinners, so you usually need to use their own-brand thinner with those two, or stick to water)

However, you do not want to dilute paint too much with solvents alone (water/alcohol etc), or you change the ratio of solvent to binder too much - too much solvent and not enough binder means the paint doesn't dry or adhere properly. Acrylic medium, such as GW's lahmian medium is a mixture of water and binder - the same as their normal paint - so you can dilute paint down, while still keeping its normal properties. Contrast medium is needed for contrast paint, as that is somewhat different to conventional paint.

When paint has dried out a lot and is very goopy, or you just want it a lot more liquid, it's better to add acrylic medium rather than just water or thinner, as some of the binder will be dried out and stuck to the pot, as well as water that's evaporated - you need to replace both. So plenty of medium to get it 'rehydrated' with a good ratio of water to binder, then some thinner to help disperse it better, or just use a thinner that also includes acrylic medium (liquitex airbrush medium is my goto here, I use it as a cheaper alternative to lahmian medium, whether thinning normal paint etc or rehydrating nearly dried out paint. I use Contrast medium for thinning contrast obviously, and I've recently switched to using it for thinning washes too, it makes them seek recesses even better while being less intense).

Flow aid is another additive; the primary purpose of this is reduce the surface tension of the water, making the paint flow off the brush more easily. It also helps break up lumpy or chalky paint. Airbrush thinner usually also includes a little of this, or you can use it on its own (in very limited amount) to help white or near-white paints in particular go on more smoothly. Again, GW doesn't use this, which is one reason their whites/near whites tends to be more chalky/lumpy than others, because they've assumed that children will sometimes drink the paint! Other manufacturers do not hold themselves to this. Do not drink paint that has things like airbrush thinner or flow aid added, obviously - even brush licking is a bad habit when flow aid is involved.

Edited August 10, 2022 by Arkhanist