• Monday, December 06th, 2010
There is more to priming than simply applying a level of stuff for pigment to attach to. Many guides place the major choice on whether to use black or white spray primer and lay the basis on that decision on whether the end result will be a light or a dark model. This is basically crap with a little bit of truth to it. If most of the model will be drybrushed and the under coat of primer will be seen and used as shadow, then a choice of black primer does directly influence the final appearance of the model. Yet, with even the most basic of paint jobs, the model will be entirely base coated with other pigments and there will be very little translucency through the layers above the coat of primer (more about translucency can be found in thee section on washing).
The main factors in choosing a type of primer are durability and absorbency. Brushable primers tend to be more durable than spray primers and have far fewer reactions to environmental concerns than sprays. The one major exception to the durability of brushable primer over that of spray is the use of black spray primer on plastic. Black primer contains an additive that causes it to bond to plastic. Plastic models primed with black primer will be very hard strip later. If striping a model of it’s paint is of concern, do not use black primer on plastic. If an incredibly durable coat of primer is most important (this, of course, would make the layers of pigment on top of the primer considerably more durable) then go with black primer on plastic. Painting on brushable primers does take far longer than spraying models, but it is possible to get consistent results without factoring in environmental concerns.
Spray primers react poorly to environmental humidity. Spray primers in the presence of humidity set up their structure unevenly. This can cause a slightly rough texture on the model or even make the final produce fuzzy. Wind can also cause this effect by partially setting the primmer before it hits the model, the result is a kind of dusty-fuzzy model whose primer coat is a mix of primer that dried before it hit the model mixed in with primer that was wet when it hit the model. It is also fairly easy to over-spray a model. Putting too much primer on a model can result in a significant loss of detail and is most often the result of poor application or interference from the wind.
Of the three major kinds of primers (black, white and gray) there is more difference than just pigment. The basis of the two different primers come form their original design. Originally, black primer was for use on plastic and white on metal. There are chemical difference, not just pigment and the pigment difference was originally to signify the primer’s usage. Black primer bonds better to plastic and was specifically designed to be a binding-primer. Since the primer is itself a polymer, it bonds well to itself and is the better choice for models with a very flat microscopic surface and plastic (which it readily bonds with). White primer produces a larger structure and bonds better to surfaces that are slightly irregular. Most metal models have surfaces that are microscopically rougher than plastic, but it really depends on the metal used to make the model, how and with what the model’s mold was made from, and even the different processes that different spin-molds use. In general, white works better on metals than black because if was originally designed to be a filling-primer for rougher surfaces. White primer is also far more susceptible to interference from humidity than black primer. Grey primer is a mix of the two and can offer some advantages in versatility.
Another area of difference in primer selection is absorbency. Because black primer is a binding-primer and white is a filler-primer, the two can have different levels of absorbency. Applied in a low-moisture environment, these differences are minute, but as the humidity is increased, so is the difference in absorbency. Because it is rather difficult to control the exact amount of humidity in which the primer is applied, using this as a reason for selecting between the two is quite strained and with modern washed is almost a moot point, but is is still a factor that can be exploited. The actual ratios of humidity to primer-absorption is well beyond the scope of this article, and would require a great deal of experimentation.
The main limitations of aerosol-based spray primers can be overcome through control of the environment in which the primer is applied and using the proper techniques to achieve good coverage and still avoid too much buildup and using layers to achieve durability. To get good coverage, preserve a model’s details, and obtain a durable coat, one needs to shoot the model from different angles and use multiple layers. One technique to achieve this is to put the models on a flat surface standing up on the base, spray a light coat, turn the flat surface forty-five degrees, spray another light coat, then repeat until the circle is complete. Let the models dry, put them on their back and do this again. Let them dry, then repeat the process with the models on their front. This will give the models several layers for durability, and with the numerous coats from different angles, give good coverage without excess buildup. Any spots missed can be quickly covered with brushable primer. A similar effect can be achieved by using poster-putty to stick the models to the flat surface so that they do not fall off. With the models stuck to the surface, the surface can be tilted to achieve the same effect of rotating the models on the surface. The putty can be reused multiple times and never again will a model suffer falling damage before it has even seen the paint table. An even better method would be to place the model in an empty paint-pot or similarly shaped object and spray them individually.
The best way to solve the environmental issue is to spray the models in a controlled environment, aka inside. To do this and still avoid the sticky question of cancer, it is necessary to use a paint-booth. The best answer for a paint-booth is to convert an entire room using industrial grade equipment. The better answer for a hobbygame nerd who would rather spend the money on models is to make a paint-booth for less than a hundred dollars. First, an outlet to the outside is needed. The best solution for this issue is to use a dryer vent, next is a window, followed by cutting a hole in the wall. Each possible solution depends on the nature of residential ownership and the ability to readily access gray-tape. Get a large moving box and assemble it. Cut off the top and one side. Cut a hole in the back that will fit an air-filter. Tape the air-filter into place (or find something that will allow an air-filter to slide in and out for easy replacement and tape that in place. Put a cheap box-fan on the back so that it blows away from the box, and tape the ever-loving-hell out of that so that the sides are sealed and it will only draw air through the filter. Next, use more tape to attach a trash bag to the fan’s other side and use more tape to connect the other end of the bag to a dryer tube. Run the tube outside through one of the above mentioned methods. Boom, paint-booth!
