Whether a newly cast figurine or freshly glassed boat hull, the silky smooth texture of cured polyester resin presents a distinctively difficult painting challenge. Most paints simply cannot adhere to the slick surface. Paint pools and if it does dry, it often chips and flakes with the slightest bump. By carefully following a step-by-step process that includes thoroughly cleaning, sanding, priming, and applying both a basecoat and topcoat, crafters and boaters can overcome the polyester painting problem and enhance the overall look of their resin projects.
Polyester Resin Properties
Unsaturated polyester resins starts as a liquid and becomes completely solid with the addition of a catalyst hardener. Most marine and figurine resins contain a small amount of wax. This rises to the surface after application and forms an airtight seal that allows the resin to cure. Without this seal, the resin does not set completely and results in tacky surface. In this case, the resin needs some sort of wax-based topcoat. The average product working time for polyester resin depends on environmental factors like temperature and humidity, but typically lasts up to 30 minutes. It cures in 6 to 8 hours.
Polyester Resin Painting Process
To begin this project, first assemble the necessary supplies. For prepping the resin surface, gather a heavy cleaner like acetone or denatured alcohol and 180-grit wet use sandpaper. Choose a quality spray primer formulated for use on resins, plastics, or metals for the priming step. The materials for painting depend on the project: Select either a weather resistant marine paint for watercraft or a gesso basecoat and acrylic topcoat for crafts. Also stock up on generally useful supplies like lint-free rags and paper towels as well as safety gear like rubber gloves, protective eyewear, and a disposable dust mask.
- Prep Surface
When refinishing older polyester resin, first inspect the surface area for damage. Use all-purpose fiberglass filler to repair and seal cracks, nicks, or other irregularities. Let all repairs cure completely. Next, thoroughly wash the surface with a wax-removing, non-residue cleaning solvent. Repeat cleaning on new resin to remove all of fresh sealing wax. Finally, sand the surface with 180-grit wet sandpaper. Continually wet the sanding area with water from a spray bottle and move in smooth, even circles. Stop when the resin surface looks cloudy. Wipe the sanding residue away or blow it off with a compressed air duster.
Prepping the smooth resin surface improves paint adhesion later, but not nearly as well as several coats of primer do. Double check that the surface is free of grit and completely dried before proceeding. Equip a can of quality primer — PlastiKote Sandable Primer or Krylon ColorMaster Plastic Primer works well — with a reusable snap-on spray paint holder. This handy tool reduces finger fatigue and makes even painting much easier. Holding the can about 8 inches from the surface, spray the first layer of primer using quick, evenly spaced strokes. Opt for finer coverage over thick to minimize drips and drying time. Let the first coat dry for about 10 minutes before applying a second layer.
Up to this point crafters and boaters can utilize the same steps for working with polyester resin, but forthcoming painting techniques differ. When working on watercraft, pour marine paint into a standard painter’s tray and use a snort nap roller for application. Paint three to four layers and consider adding an additional layer around the water line and other spots where the boat suffers the most wear.
Crafters should first apply a gesso basecoat to their resin cast or composite. The product is typically used to prep canvases before painting, but when used on primed polyester resin it provides an additional adhesion layer that paint easily sticks to. Paint a single thin base coat of gesso onto the cast with a synthetic bristle brush. The product goes on thick, so keep a clean, damp brush nearby to sweep away any gesso globs before they dry. When dry, paint over the base coat with standard acrylic paints.