Preparing Resin Parts For Painting

06 February, 2012 5 comments Leave a comment

 All of the 1/64 / HO slot cars, figures and structures,1/72 diorama kits and accessories, etc. sold under the Full Circle Hobbies name are cast in a high quality polyurethane resin. Resin is a type of plastic used for low volume production of parts and possesses some very different procedures for preparation and assembly compared to plastic.

 The first step in the preparation of resin parts, is to sand any parting lines and / or  pour plugs. Use a dust mask while sanding resin, as the fine dust created can be irritating if inhaled. With small castings, cut the ends of the pour plug as close to the part(s) as possible with a small wire cutter or nail clipper. Grind off the remaining pour plug using a Dremel tool fitted with a sanding drum attachment. Grind the pour plug until it's paper thin, then use an X-Acto knife to trim the remaining resin from around the part(s). Sand with sandpaper or a sanding stick until the part(s) are the correct shape.

 With my resin cast slot car bodies, I cast them a bit thicker than plastic bodies to help with the casting process. This has another advantage, as it allows you to fine tune the handling of the car by grinding the inside of the roof thinner to lower the center of gravity. Also, by grinding all of the inside thinner, you can make the car as light as you want.

 Sometimes warm temperatures during shipping can warp resin castings. If you encounter this problem, don't worry. It's very easy to straighten resin slot car bodies, figures, or small diorama parts. You can use a hairdryer or a heat gun set on medium to slowly heat the area that's warped. Heat the affected area until the resin becomes a bit soft, then slowly bend the casting until you're satisfied with the straightness. You might have to repeat the process a couple of times until you get the results you're looking for. Do not force parts to bend. As the part is cooling, you can make small adjustments.

 Another way of fixing warped resin parts, is to let the parts soak in VERY warm water. This will soften the entire part, and will have to be monitored carefully so the parts don't soften too much. 10 - 20 minutes should be enough.

 To clean release film from castings, soak the parts for 30 minutes in warm water with a bit of dish soap, then wash them thoroughly with dish soap under warm running water using a soft 1/2 inch wide fine hair paint brush. The brush will let you work the soapy water into all of the hard to reach areas. If you soak the parts in water that's too hot, the resin parts might soften and warp. In the past, resin casters had to use a mold release agent that left a heavy oily coating on the cast parts, but now most manufactures including myself, use silicone self - releasing molds that leave parts almost clean. It's still a good idea to wash the parts just to make sure your paint will stick. If you don't wash the parts properly from some manufactures, paint will not stay on them.

 If soaking the resin parts in dish soap doesn't remove the mold release coating, use acetone. This can be found at most hardware stores. Pour the acetone into a small glass container, and place the resin part into it. Immediately scrub the part with a soft, fine hair paint brush getting into all of the small areas. Take out the part, wash with dish soap under warm running water, and let it air dry on a paper towel. The mold release should be completely removed by using this method. One word of caution. Don't use any plastic containers, or brushes for this cleaning technique. The acetone will dissolve plastic.  

 Unlike the styrene plastic used in the typical model kit, resin is impervious to most solvents and paint thinners. Just about any type of paint or primer, automotive, home, or hobby paint, either lacquer, enamel or acrylic, can be used on resin without fear of crazing the surface. Use white or grey primer made for resin, metal, and plastic for slot cars and figures, and flat (matt) black primer made for resin, metal, and plastic for dioramas and buildings, as this also acts as a pre-shade. Primers that are not made for resin will not bond to resin castings!

 On some of my large diorama parts there's a thin lip around the casting. This is a sanding guide. Tape a large piece of 80 grit, or coarser, sanding paper to a flat surface and place the part on it. Sand slowly in a circular motion until the guide is paper thin or sanded off completely. A darker primer coat makes this process easier to see.

 Once the parts have been sanded and parting lines and / or pour plugs have been removed, you might need to fill some casting imperfections or small air bubbles. These imperfections can be filled with automotive spot putty which comes pre-mixed in a tube, or your favorite modelling putty.

 Once all of the imperfections have been filled and re-primered, it's time to glue the parts together. Sand or scrape the primer off any mating surfaces before gluing them to ensure a strong bond. The best glues for bonding resin to resin or resin to other types of materials, is 5 minute epoxy or CA glue. Do not use CA glue for bonding clear parts, as this type of glue will cause fogging of the part. Regular glues made for plastics and wood will not bond resin permanently.

 After all of the above has been done, you can now move onto the most fun part of model building, painting and adding details. For information on painting models, read my series of articles on this site. You'll find them very helpful.

 I hope this information helps you with preparing resin kits for painting. If you have any questions or comments regarding my products, please don't hesitate to contact me at fullcirclehobbies@hotmail.com.

 Happy model building!

David Gurinskas / Owner of Full Circle Hobbies