Testing out Auto Paints & Primers on PP and PS plastic

 Sometimes a seemingly inconsequential circumstance (the case in point being my inability to find a specific hobby paint color i.e. Tamiya Mica Blue for some recently acquired car scale model kits) will kick off my inane hobby OCD behaviour. With so many miniature projects pending you might think I would just put sourcing the out-of-stock paint on the back burner and concentrate on the huge pile of ongoing projects. But no. Cue a search online for an alternative, resulting in a significantly cheaper yet not necessarily wholly ideal solution. Let me explain ...  

Spray Man (Subaru WRX OC2 Blue) automotive grade paint on a polystyrene spoon

 My quest for an alternative paint began with a search for an extremely specific hue i.e. the dark blue metallic hue that's iconic to the Subaru Impreza WRX STI. So my primary concern wasn't the type of paint available but more towards the color of the paint. Any worry of whether the paint is suitable for scale model kits became secondary to color accuracy. Subsequent research paid off in the way of automotive grade primers/paints as an alternative to the plastic-friendly hobby lacquer primers/paints. These are the Spray Man Automotive Grade Paints and Primers you see below.  

Spray Man Automotive Primers, Paint, and Clear Coat

 It has been quite a while since I used aerosol spray paint cans, and I was quickly reminded about how messy things could get while using a spray paint can. Naturally, I made a right mess of things and the cement floor at the front of the house now has Subaru WRX STI blue paint on it. Oops! Meanwhile, a unique feature of this particular brand of automotive spray can is that it comes with two different types of interchangeable nozzle. One sprays out a flat horizontally-shaped layer of paint while the other sprays out a circular-shaped layer of paint. The latter is what you would normally find on hobby spray paint cans. In this test, I used the flat-horizontal spray nozzle.  

Applying automotive grade paint via a spray can onto a primed plastic spoon

Cardboard boxes and pieces used to prevent paint over-spray from damaging the house ...  

... but unfortunately the missus won't be best pleased with the results of my overzealous testing

 On what surface were the automotive grade paints/primers tested on? Well for the tests to be effective and useful, the paints had to be sprayed onto plastic that were as close as possible to the types found in scale model kits. Unless you're using spare plastic sprues or parts from the model kit in question, it is almost impossible to 100% match the type plastic found in my test plastic spoons. For example, even though most model kits use Polystyrene (PS) molded parts, the characteristics of said parts differ from brand to brand e.g. some brands have softer PS parts than others.  

Plastic Recycling Symbols for Polypropylene (PP) and Polystyrene (PS)

 In my case, I have scale model kits of the Subaru Impreza WRX STI from two brands, namely Tamiya and Aoshima. As I understand it based on a perusal of plastic symbols found in the Japanese language box art and instructions, Tamiya uses mainly PS plastic while Aoshima uses PP plastic. For the closest comparison I could muster, I used PS and PP plastic spoons to mimic Tamiya and Aoshima molded parts respectively. [Caveat: While the spoons and molded parts may comprise similar types of plastic, their specific characteristics may differ slightly from one another.]   

Polypropylene and polystyrene plastic spoons I had saved from takeouts and visits to eateries

 Prior to achieving a successful outcome of sorts, there were plenty of failures along the way. The one common theme of said failures is the ineffectiveness of lacquer-based hobby primers be it Mr.Hobby Mr.Surfacer/Mr.Fine Surfacer or Tamiya Fine Surface Primer in withstanding the application of either an automotive grade filler primer or paint, when used in conjunction with PS plastic spoons. For automotive grade paints to work on PS plastic spoons, first an automotive plastic primer needs to be applied on the spoon, followed by an automotive filler primer. Once both primers have been applied, the automotive paints will adhere to the PS plastic spoon fairly smoothly.  

Experimenting with Hobby Primer and/or Auto Filler Primer/Plastic Primer on polystyrene spoons

Testing Auto Paints on Mr.Surfacer 1200, Tamiya Fine Surface Primer, and Auto Filler Primer

 Using the two-step priming process (i.e. automotive plastic primer followed by filler primer), the application of automotive grade paints also work when used in conjunction with PP plastic spoons. But there is a significant advantage to using PP plastic. As you can see (in the photos immediately above and below this paragraph) PS plastic spoons primed directly with automotive filler primers do not exhibit the necessary surface texture that allows the automotive grade paint to adhere properly.

However, when PP plastic spoons are sprayed with only hobby-grade primers or directly with automotive filler primers (sans plastic primer), they exhibit the required surface texture that allows the automotive grade paints to adhere perfectly to the PP plastic. I do not know the science behind this, but if I had to hazard a guess, I'd say that the PP plastic is strong enough to withstand the corrosive nature of the solvents that exist in an automotive grade paint/filler primer.  

Testing Auto Paints on Mr.Fine Surfacer 1500, and Auto Filler Primer/Auto Plastic Primer

 Even with the eventual success I achieved when using automotive grade paints on PS plastic, it does come with two important caveats though. First, the use of a plastic primer followed by an auto filler primer creates a surface that is sometimes not 100% smooth to the naked eye. I've yet to consistently and easily create smoothly primed surfaces using this two-step process. Maybe I'll perfect the technique in the future, at least I hope so. Second, having to first apply a plastic primer before a filler primer creates unnecessary thickness to the final paint coat. More so when you consider that a final clear coat will need to be applied for optimal results.   

Success with auto paints on polystyrene plastic spoons requires specific steps

Success with auto paints on polypropylene plastic spoons requires less stringent steps

 So at the end of it all, the choice of whether to use auto paints and primers on scale model kit plastic boils down to one thing. Is color accuracy paramount in your hobby project? If the answer is yes, then go for it. I believe it's a risk worth taking. If not, then I say you're better off with using hobby primers and paints, which are more benign on plastic parts. Then again, PP plastic seems to work well with automotive grade paints and primers. But very few scale model kits I know of actually use PP plastic for its molded parts. Even for Aoshima kits, I'm not entirely sure if all or just some parts are PP plastic. I'll know for sure only when I use the automotive grade primers and paints on the parts itself. That's a challenge for the future. As for now, it's time to end this post. Stay safe, and be well!