Resin castings need to be properly prepared for painting to ensure the best possible quality in the finish job as well durability over the long term.
It is important not to cheat the prepping process! The appearance of the finished resin depends  upon what you do in these preparatory stages, and a model will end up to be less than it should be due to lack of a few more minutes of attention!
Typically, to prep, prime and basecoat a resin can take from 30 minutes to 3 hours, depending upon the size, seam locations, detail level and overall quality of the casting. Poorer quality castings or those with a lot of intensive fine detail may require additional time.
The first step in preparing a resin is to clean off the seams and any other flash or excess material remaining from the casting process. Seams can be removed by several means, including:
- X-Acto knife
- An x-acto knife can be used to carve out areas that can't be reached with any other tool as necessary.
- Carbide Scrapers
- These specialized tools are among the most effective means for removing most of the seams from a resin model, and will allow you to carve in detail along a seam as you go.
- A light application of sandpaper after the major areas of flash are removed will result in a flawless finish. (Wet-or-dry 320 or 400 grit is recommended to avoid introducing visible scratches to the piece)
Sandpaper can be used to remove most of the seams if you have no other fine tools available
To smooth out gouges or dings, or small depressions along seam lines (or anywhere else) that lie below the intended surface of the piece, here's a few great tips to help...
- Trick No. 1-- Super Glue and Baking Soda
For bubble holes, pits and larger areas, this is the best method. Depending upon the nature of the area to be filled, you may wish to experiment with different techniques. For shallower holes, fill the hole with liquid super-glue (do not use gel as this technique does not work with gel-type glues) then drop a pinch of baking soda onto the glue. The glue should turn white and harden instantly. Often, the soda-glue bond area will raise above the surface to be filled. This can be leveled off with sandpaper easily.
For deeper holes, it may work better to pack the hole with baking soda, then add a drop or two of liquid super-glue to the soda.
If you still have a depression in the area, just add a little bit more glue to the area and drop more soda onto it.
Skillfully applied, the soda-glue bond method can be used to fill in missing areas on eartips or other thin areas where there may have been an air bubble in the casting.
- Trick No. 2-- Spot Putty
Bondo or Loctite brand red spot putty for automotive body repairs works very well to fill in shallow surface dings and small open areas. Use a small palette knife to smear the putty into and over the area. Spot putty does not work all that well to get into very small pinholes, but works great for more open blemishes.
This putty sands silky smooth very easily, using wet-or-dry sandpaper (320 to 400 grit). Since the spot putty is softer than soda-glue, it should not be used for deep holes or bubbles. It is the filler of choice for shallow surface imperfections though, because the soda-glue has a tendency to build up over the surface of the piece when it solidifies, and since it is harder than the resin itself to sand, takes much more effort to smooth out than spot putty.
- Trick No. 3-- Modeling Paste
If you are faced with a casting that has a large number of pinholes over an area, brushing modeling paste over the area and smoothing it over with a brush is a quick and effective way to deal with it. This is particularly true for pinholes within the mane or tail of the sculpture, or sculpted feathering on the legs.
To make sure all dirt, oils and any mold-release residue from the casting is removed, and to add some "tooth" to the surface to hold primer paint better, the easiest way to do all these things at once is to "ajax" the model. This can be a little bit "messy" even for a cleaning product, so dress appropriately.
Start by adding a little water to some ajax to make a "paste". Apply this paste with an old toothbrush to the model and scrub the model thoroughly all over its surface. Don't miss any nooks and crannies where mold release, grease or dust may hide out. A good ajaxing job on a traditional-scale (1:9) model will take about 5-10 minutes, depending on the mold and how quickly you proceed.
When the piece has been thoroughly scrubbed, rinse off the model and set it aside to dry.
Resin compounds and automotive body products are designed to work in tandem with each other. Spray primer is mandatory at this stage, regardless of how the model will be painted or finished off in the end, as it has the best overall adherence to the resin, and also seals it properly. Krylon (or other brand) Sandable Primer works well for this purpose.
Using spray primers is best done outdoors to get adequate ventilation. Appropriate clothing as well as latex gloves for easy cleanup of your hands afterwards is highly recommended. Try to do your priming on a warm sunny day when it is not windy. The primer will dry quickly this way, and allow you to proceed faster than when the weather is cooler (under 60 degrees) or damp.
Always be sure to shake the can well to mix the paint inside up properly. If the primer is not thoroughly mixed, it can result in "crackling" on the surface (which will cause you more work).
Spray Primer is generally available in only a few colors, white, grey and ruddy brown (red) are the most common. (I recommend using white for buckskins, duns, palominos, greys, red for chestnuts and bays, and grey for blacks).
Apply the primer evenly over one half of the horse (presumably you will be holding the horse by the other half). Don't apply so much paint at once that it drips or sags and be sure not to get the nozzle too close to the horse. Several lighter mistings are usually better than a thick heavy coating. **
**thick heavy coatings take much longer to dry and can remain slightly "soft" or "gooey" for a day or more. While "soft" they are easily gouged and will cause more work. On occasion, you may wish to use a thick heavy coating of primer to help fill and smooth the surface of a resin you may feel is overly rough in an area. Sometimes this is more effective and quicker than trying to sand such areas. Keep in mind that its very easy to get runs and sags using thicker paint, and you need to plan to allow the primer to dry a couple of days or so before you can continue.
Allow the horse to dry awhile, until you can safely pick it up by holding the primered end, and then spray the other end.
When the primer is dry and you can safely handle it, inspect the surface for any gouges, dings or seams that were missed. The better you do on the initial seaming portion, the less leftovers you should find at this stage.
Raised imperfections can be lightly sanded off with 400 grit wet-or-dry sandpaper. The paper will gum up far less if its used wet on the primer.
Fill any small dings with spot putty and when the putty is thoroughly dry, sand the areas smooth with 400 grit wet-or-dry sandpaper. Wetting the surface before you start will not only reduce dust, but will sand faster and smoother without gouging the primer.
If you find the sandpaper is gouging the primer, the primer is not yet dry enough to work with. Such gouges can be sanded out later when the primer is fully dry in most cases, and if not, another coat of primer over the area, followed by a little sanding should do the trick. Spot putty can be applied to a serious gouge if necessary.
Be very careful at this stage... it can be all too easy to add more imperfections than you are fixing if you are not careful! Proceed slowly, with care.
When the model once again appears to be smooth, apply another coat of primer. Allow the paint to dry, then check the surface again for imperfections, and repair them as before. Continue the process as necessary until you feel you've obtained the best surface possible.
At this stage it is best to let the model sit a day or three to allow the primer to dry and harden fully.
Some people like to paint directly on top of the primer, while others prefer to basecoat the piece with gesso or acrylic colors. You can handbrush gesso or acrylics onto the piece, or apply them with a fine-celled sponge (for a slightly textured surface). Additionally, an airbrush may be used to apply a basecoat. Keep in mind that gesso, in particular, tends to be thick and can build up on the surface quickly. If you are handbrushing gesso onto the surface, apply it in several thin layers to prevent buildup.
Each artist has their own preferences for the painting surface they prefer, smooth or textured, white gesso or colored basecoats, flat or shaded... if you are an artist new to this type of three dimensional painting, you may need to try experimenting to find out what works best for you.
Once a basecoat has been applied, the resin sculpture is now ready to be officially painted its actual finished color, in oils, acrylics; by handbrushing or airbrushing.
- If you need to rebuild a sizable area that is missing from a resin (a tail tip, ear, nostril rim or portion of a hoof are the most common areas to have problems) using epoxy putty may be more effective than using soda-glue bonding. Martin Carbone "Gapoxio" putty works very well for this purpose when you need to "sculpt" a missing or damaged segment rather than just fill in a hole.
- For models sold as "hair prep" (which include no sculpted mane and tail, and are designed for the finishing artist to add either a mohair mane and tail or to add a sculpted version) epoxy putty is the best and most durable material to use for sculpting manes and tails.
word of note in regard to resin sculptures....Many resin horse bodies are produced by artists with the intent that others will finish and complete the pieces by painting them (or possibly adding sculpted or haired manes and tails to them if they were designed as such). Since the original sculpting artist retains the copyrights to the sculpture form itself, they also have the option to set policies regarding what they do or do not wish to have done to the sculpture work.
While many artists feel customers should feel free to modify the sculpture and have fun, others may prefer that the sculpture itself is not modified. Yes, it is common courtesy to adhere to the artist's wishes in this respect, but also federal law indicates that the artist may choose to allow or disallow modifications to their work.
If you are ever in doubt about these issues, simply ask the artist if they have any objections to modifications of the sculpture. Some artists may post their policies in their sales information, or on a web site, or include them along with the piece.