Sunday, November 8, 2009

You Crack Me Up!

I was surfing the FTW links and saw this post by Drathmere.
I have experienced this a few times and while I commented on it over there I thought I would throw them up here for anyone else having similar troubles. So one with the show!

Contraction cracking, another bane that must be dealt with by painters.
Traditionally there are a list of issues that paints can suffer from, some which are inter-related, and these are below:
At the top of the traditional list are; Exposure, Elasticity Loss, Elastic Failure, Paint Adhesion Loss, Paint Blooming and Paint Blistering.

Paint Blistering is an easy one. It can be caused by a couple of things, most notably dirty or no properly prepared surfaces and moisture. Most of us are familiar with the two things we need to do before painting. First wash the Mould Release agent off, and then let the mini dry. These two steps will allow you to avoid paint bubbling 95% of the time. With resins it is also important to use a good primer. Also Using painting stands rather than holding the minis yourself will stop any oils from your fingers causing finishes and layers bubbling. All familiar things so one to the next topic.

Now we are all familiar with the Dreaded "Frosties!" and according to the painting dictionary this is called Paint Blooming.
"Paint Blooming - (blushing) - this is moisture getting into varnish, shellac, lacquer. This coating failure leaves a milky opaque (usually-white) cloud on (or actually within) the coated surface. Paint bloom may not wipe off with a fingertip as does most effloresence - so paint blooming is not the same visual effect as surface effloresence, many paint failure analysts use "bloom" for both cases and the cause might be the same - moisture."
Paint Effloresence is frosting on the surface that you can simply wipe off btw.
In some cases the Paint Frosting can be caused by Chemical incompatibility with the surface, this is very rare in miniatures as the traditionally used Matt Finishes we use are formulated for the job.
This is the topic of many many discussions and we all are familiar with it so I will move right along.

Exposure and Elasticity Loss don't generally effect us in Miniatures so I can probably skip these.

Elastic Failure is the point at which paint can no longer stretch and will crack. This can be cause by alot of things including paint too thick, many layers of paint and low elasticity of the paint. This last is important as generally we will use acrylics with do not have polymerisation compounds that allow them to stretch that little bit as they dry. This can lead to cracking very easily. Adding things like Paint Extender and Drying inhibitors can also change the elasticity of a paint, sometimes for the better (I notice that adding a little Retarder Medium to Reaper paints helps prevent them cracking when used over semi-gloss/gloss undercoats). This can also be related to Adhesion Loss.

Adhesion Loss is a big one and is when the top layers of paint loss their grip on the under-laying coats. There are a few causes of this. A big one is improperly prepared surfaces, this is caused by the same factors as Paint Blistering. Another primary cause is using incompatible paints such as Oils for automotive use under or over acrylics (in which case the solvents in the Oils will do nasty things all around). Using material specific paints like the PS line from Tamiya will also cause issues as they are formulated to peel of perspex. The last and most common reason paint will crack is using acrylics over a semi-gloss/gloss spray-packed undercoat. The top layers of paint will often have difficulty sticking to the smooth gloss finish underneath and will have a definite tendency to pulling away and cracking as the Matt Finish on top contracts while drying.

So what tips can I offer on possible fixes specifically for Paint Cracking:
First and foremost use a good primer; I use a light spray of Tamiya Light Grey "Fine" Surface Primer for Metal and Plastic before undercoating. It will even work on Resin. Be absolutely certain you get the "Fine" version and be very light handed with it as it will pool in detail really easily.
Also if you use a semi-gloss or gloss undercoat spray (like Tamiya Racing White or their Blue that is the same colour as Necron Abyss) you can try using a matt sealant after undercoating to stop the thin overcoats moving and chipping.
If you are using a semi-gloss, buy a slightly lighter tone than you want and once it dries wash the whole things with a pigment based wash with a little bit of dish-washing liquid in it, it helps and will play a dual role of adding down-tones to your base coat with very little work. I am going to guess that the Dish-washing liquid breaks up any volatiles left on the surface of the spray-pack but don't quote me on this.
A neat thing I found out was that you can use the Tamiya Perspex paints (the PS range), that will normally peel off (designed for painting inside the clear racing shells for remote control Racers). All you have to do it spray it on, let it dry and then spray some purity seal over it. There are some really different colours in the range that you don't normally see but are Great for really odd colour schemes.
Use thin coats and let them dry completely for at least a full day before Matt Finishing them. If you have trouble with a specific brand of paints try switching to another brand, there are a lot of companies making good paint, so experiment.
Finally sometimes you can get around the issue by using a Gloss Clear Finish first and when it dries a Matt Finish over the top. Alot of Gloss finished don't contract as much when drying. I recommend testing it out first.

Best of luck with your painting endeavours
Subby

2 comments:

  1. Great post! I've been thinking about using grey primer for a while, and this post kind of tips the scale. After reading this, I wonder if my shadowsword frosting was caused by spraying the dull coat in the same box that I had airbrushed it in. The moister may have been artificially higher. I am not sure I could have done anything about the paint cracked except using more paint to produce a stronger coat. I appreciate this post Subby!

    ReplyDelete
  2. Drath,
    another trick you can try is something I do when I really need to Dullcoat something, turn on the reverse cycle aircon for an hour or two and spray in the room with it on. It will strip the moisture out of the air (this is why your eyes will get sore and dry in office buildings!). Also make sure the spray tin is at recommended temp (even if you have to warm it up). Otherwise this can cause funny things to happen as the aerosol changes pressure while being sprayed.

    ReplyDelete