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Trouble with Basecoat


vicwiz007
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"Paint" is pretty much a mixture of three things. There can be various additives, but for simplicity in description, it consists of:

  • A thinner of some sort. With the acrylics we use, this will usually be water, but other choices are possible, and if you're not using acrylics, there are many possibilities.
  • A monomer. This is the plastic precursor chemical that will form the final film on the surface. Monomers ("mono" = one) link together with others of their species to form polymers ("poly" = many) in much the way that chain links connect to each other to form the actual chain. The resulting polymer will be clear or translucent, since otherwise you couldn't see the color of the next part.
  • One or more pigments. These are the chemicals that give you the color of the paint. They're suspended in the paint like mud in a turbulent river. Note that this is why you need to shake your paint, because they're not dissolved in the thinner, they just can't fall to the bottom very quickly. But note that when a river enters a lake and slows way down, that mud will fall out of suspension and drop to the bottom of the lake; paint does the same thing over time when it's not agitated. Note that very dense and large grained pigments can fall through the thinner to the bottom of the pot more quickly, which is why some paints are more prone to settling out than others. E.g.: Titanium Dioxide, the most-used white pigment, is both dense and relatively large grained (grain size is critical to get the color; all Titanium Dioxide paints will have pigment grains of very similar sizes.)

Paint "drying" is a combination of two things:

  • The thinner must evaporate, since it's keeping the monomers apart and preventing them from polymerizing, or chaining together to form a plastic film. This is strongly endothermic (requires a lot of energy to happen).
  • The monomers must crosslink to form polymers, which is an exothermic reaction (releases energy when it happens).

So when the paint is dry to the touch, much of the thinner has evaporated, allowing the monomers to come into close contact and start forming polymers ("When a mommy monomer and a daddy monomer love each other very much, ...." ::P:). Those polymers lock the pigment grains in place, giving the final colored paint film. But the polymerization process takes some time, so until that process runs to completion, the paint film is very fragile. Because the surface dries out first, the polymer will form first as a very thin film. If you disturb that film mechanically before it can strengthen (say, by vigorous drybrushing or adding lots of thinner before the film is fluid tight), you can break the paint surface.

 

Note that if the substrate has "stuff" on which the paint is floating (many chemicals like finger grease, mold release, and plasticizers don't mix with water, so the paint will tend to float above the surface on a microscopically thin layer of whatever), the mechanical strength of the film will be very low and subject to mechanical damage. Hence the need to clean the figure first. That said, there are some paints that are more miscible with those chemicals than water-based acrylics (like rattle-can enamel paints), so those can work better to make a base layer that adheres well to the substrate.

 

Unfortunately, those same rattle-can primers can also interact badly with some of those chemicals preventing paint from ever setting or adhering properly. But since the chemistry of each of those primers varies, it's very difficult to make categorical statements about what will work well with what kind of miniature without extensive testing.

 

1 hour ago, Wren said:

I have heard that heat helps curing. Not 100% sure if that's true, but I'll take a few minutes and use a hairdryer on a primed mini if I have to paint immediately after priming.

 

Heat increases the rate at which the thinner evaporates, since that's so strongly endothermic. (This is why sweat cools you off on a warm, dry day, since it's pulling energy from your skin in addition to pulling energy from the air.) So adding hot, moving air adds enough energy to get that water (or whatever) out of the way quickly.

 

For the actual polymerization, higher energy means more Brownian motion so the monomers smash into each other more often. (Think "atomic level mosh pit".) So it increases the chance of the right parts of molecules coming into contact in a given time. At very high energy levels, it can also smash things together enough to break bonds, but at the energy levels provided by a hair dryer, that shouldn't be a major issue.

 

The one concern would be driving off the surface thinner very quickly and building a skin that prevents the thinner deeper in the paint layer from evaporating quickly. This can be a problem with very deep paint layers. If you use thin layers (not necessarily heavily thinned layers), this is mitigated.

 

But I'll also mention that I've had some success (with metal figures ONLY) putting painted figures into the oven for half an hour or so at 200°F. This seems to allow the paint to form a very hard polymer layer, reducing damage from travel and play. If you do this, you are doing it ENTIRELY at your own risk. It's pretty easy to forget about that plastic weapon you used or the glue that can't handle the heat or the base that disintegrates on the baking pan, or whatever. I do not warrant any positive effect from this.

 

1 hour ago, Wren said:

By let dry, I mean the paint is touch dry and no longer shiny at all. I use a hairdryer if I'm being impatient and things are taking a while. I have never waited an hour after painting the first coat of a basecoat before painting a second unless I got interrupted during painting. :->

 

I don't generally wait for the next layer much either for a couple of reasons:

  • I live in a near-desert, so paint drying happens very quickly here, relative to a swamp like (say) Houston.
  • With thin layers of paint, polymerization happens pretty quickly anyway, and you really don't need that much film strength unless you're dry brushing or putting down a lot of thinner in the next layer.

But when I start seeing paint being picked back up with the subsequent layer of paint, it's time to step away for a few minutes and allow the film to toughen up a bit.

 

Other things based on paint chemistry:

  • Adding extra stuff to the paint can interfere with the polymerization process. So adding a drying retarder (so you can work the paint longer) or a wetting agent (flow aid/surfactant/detergent) to get the paint to flow better should be done with care. If you read the instructions on the bottles (I know, I'm a tech writer and I often don't either :B):), you'll see that there are comments about maximums to prevent the paint from being damaged. Be careful with what and how you add these things.
  • If you add too much thinner, you can separate the monomers in the paint enough that the polymer film can form unevenly. (This is sometimes referred to as "breaking" the paint.) The result is uneven coverage and very fragile paint layers even where there is coverage. To avoid this while still getting less pigment on the model, add acrylic medium (paint without the pigment) instead of or in addition to adding more thinner. The added monomers will allow efficient polymerization while still reducing the amount of pigment per area (allowing glazing, for instance).
  • Paint is naturally glossy, since the polymer film will generally form in very two-dimensional layers. To make the polymer film less glossy, it needs to be roughened up (with a matting agent like talc). This has a couple of major effects: 1) Since the surface is rougher, it's more susceptible to mechanical damage (from rubbing, for instance); 2) Since you are adding more stuff into the paint besides the clear polymer and the colored pigment, matte paints are inherently less saturated than gloss paints.

Finally, note that I'm not a paint chemist, so it's possible that some of the details above are not completely correct. I welcome the input from anyone with actual training in thin-film polymerization.

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1 hour ago, Gadgetman! said:

Did you use any varnish or sealer on the mini after painting?

No this was one of the first I painted. About to order a gloss and matte varnish actually :)

 

Thanks @Wren for the advice. I definitely watch other people too much. If there's one thing I learned from actually painting, it's that you can't replace practice with study. It certainly helps to watch others and research supplies etc, but you just never know what will happen until you try yourself. It's something many of the youtubers actually mention; the 90/10 rule according to Tabletop Minions channel. I'm starting to develop my own process, and learn what tools/techniques work for me.

 

And thanks @Doug Sundseth. As a current college student in a STEM major, it was a very interesting read =D I guess, as with everything else, there is a deep science to this. The more you know the better.

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1 hour ago, Doug Sundseth said:

 

 

But I'll also mention that I've had some success (with metal figures ONLY) putting painted figures into the oven for half an hour or so at 200°F. This seems to allow the paint to form a very hard polymer layer, reducing damage from travel and play. If you do this, you are doing it ENTIRELY at your own risk. It's pretty easy to forget about that plastic weapon you used or the glue that can't handle the heat or the base that disintegrates on the baking pan, or whatever. I do not warrant any positive effect from this.

 

 

 

 

I've never bothered with this for paint, but a long standing trick for epoxy is just putting it under an incandescent light. Even the heat from having the blub 3-6 inches away is enough to drastically effect the cure time. It would probably also be effective on acrylics while being much safer than the oven for plastic minis.

 

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Also, be aware that different brands of paint react differently to the bare bones material. 

 

I find that Reaper, Vallejo, Scale75 and Citadel generally stick to (washed) unprimed bones without much ado, but e.g. Army Painter beads up something horrible.

 

This probably is a function of the quality of the paint, as Army painter is considerably less expensive. 

 

After a undercoat/primer is laid down, this is no longer a problem (which, after all, is the main function of the undercoat -to key the surface to accept paint readily).

 

 

Also, be aware that several types of spray (rattlecan) primers react badly to bones, never drying properly and leaving a sticky surface for ever. There are threads in this forum that discuss this extensively.

 

I find that Army Painter and Citadel sprays do not have this problem. The last few yeares I have used Vallejo airbrush primers (black, grey, white, coloured) on Bones without any problems.

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