i recently just finished my nighthawk and did a slack ass job throwing a coat of black spray paint on my nighthawk tank... it started peeling about 1 day after i started riding haha..
what are your guys' experiences with painting plastic?
any processes that have proven to be successful? im thinking about dipping it like they do with motorcycle helmets.. like so
Different plastics require different preparation for paint. Some plastics are difficult for paint to adhere to. Due to the physical properties of various polymers, most motorcycle gas tanks are either made from high-density polyethylene (HDPE) or polypropylene (PP). They are used because of their chemical resistance and they also have good moisture-vapor barrier properties (MVTR). These two properties prevent the gas from evaporating and the plastic itself from dissolving from prolonged contact with the plastic.
Motorcycle helmets are generally made from polycarbonate (PC) which has better paint adhesion (and impact resistance). Polycarbonate requires less preparation and can more easily be painted.
The molecules in HDPE and PP have what is called a low "surface tension" which makes adhesion difficult. That's why they are difficult to glue too.
One way to prep HDPE and PP for decoration is to flame-treat the surface using a propane torch with a wide flaming head. You want to wave the flame over the surface quickly so the flame just "licks" it without melting it. This increases the surface tension by bringing carbon molecules to the surface and paints and inks will then adhere a lot better.
You can test the "dyne level" of the treated surface by dipping it in a tub of water and pulling it back out. The water will bead-up and just roll off an untreated surface (like a freshly-waxed car) but on a treated surface the water will stay more in sheets and not run right off (like an unwaxed car).
Another way to surface-treat olefins like HDPE and PP is to plasma-treat them but most people don't have that capability at home.
Both of these techniques allow paints to adhere due to a change in the surface of the plastic on a molecular scale. That's the trick to making paint stick long-term.
If you are uncomfortable with torching your gas tank, you can also sand the surface and use special primers like are used by body shops to paint PP spoilers and front fascias that some cars have.
The best paints to use on HDPE and PP are the flexible polyurethanes.