Obviously kits consist of individual parts. But the object of finishing them is to make them look like a solid object. The best way to do that is to eliminate gaps between the pieces. Sanding will take care of minor problems. However, if the parts don’t quite meet or are mismatched, filler may be needed.
Putties remain the go-to for this kind of work. But there are several types and they can be useful in different ways. Let’s look at three main types and how to use them.
Solvent-based putty
Stinky and sticky, these putties contain organic solvents like toluene and acetone. Like plastic cement these solvents can melt styrene, so they adhere well. They’ve been the gold standard for filling gaps pretty much since modeling became a serious hobby and are still widely available today. Most model or paint companies sell a version in a metal tube like toothpaste. (Don’t confuse them: Putty’s good for gaps in plastic, not for cavities in teeth!)
Before using solvent putties, especially if they’ve been sitting around, massage the tube to mix the chemicals, which can separate as paint does.
Squeeze some of the putty onto a scrap of styrene or a glass palette.
Replace the lid immediately after, wiping putty from the opening; dry putty can prevent the lid from closing properly allowing the tube’s contents to dry prematurely.
Apply the putty to the seam to be filled with a flat object.
Apply more than is needed to fill the gap, as most solvent-based fillers shrink as they dry.
You can minimize sanding by scraping some of the excess away while the putty is wet. But leave some on either side and never scrape it flush.
Most solvent-based putty requires several hours to set hard in preparation for sanding; I usually leave it overnight.
The major difference between brands is the size of the particles in the putty. Larger grit won’t sand as smooth, whereas finer ones easily feather at the edges.
Use care when working with solvent-based putties. The fumes can be harmful if inhaled, and the products are flammable. Always use them in a well-ventilated space away from flames. Also, wear gloves to avoid skin contact with the chemicals. Before the putty dries, clean tools with lacquer thinner.
Water-based putties
A recent addition to the modeler’s gap-filling arsenal is water-based putty.
Unlike their solvent-based cousins, these putties have little odor, can be handled with fewer precautions, and rarely need mixing before use. The consistency of toothpaste, they apply in much the same way as others.
But there is no need to apply excess; they don’t shrink as they dry. You can use a flat tool to spoon them over a gap. Or, because of the thin consistency, you can squeeze them directly into a seam.
Excess can be removed with a wet finger or cotton swab, which makes water-based putties useful for seams like wing roots. After applying putty, run a damp (not wet) cotton swab along the seam.
Once the putty dries, the result will be a perfectly filled and blended seam.
Water-based putties dry quickly; small amounts (as in a seam) can be sanded within an hour.
Epoxy putties
Developed for plumbing repairs, epoxy putties such as Milliput, Green Stuff, and Apoxie Sculpt comprise two parts that activate when combined.
Take equal amounts of each part using a clean tool for each; avoid contaminating one with the other and kicking off the reaction inside the container.
Knead the parts together until the color is uniform — unlike a good steak or cake, you don’t want marbling! You don’t need to wear gloves for this process, but it will keep your hands clean. If you choose not to wear them, wash your hands after mixing to minimize stickiness.
Take a little of the putty and push it into the gap.
Epoxy putty adheres well enough, but it can be helpful to rough up the surfaces to ensure it stays put. Wet your finger to smooth and shape the putty. It will take several hours to set, giving you plenty of working time. Epoxy putty works for standard gap-filling, but it comes into its own on figures where the ease of blending and sculpting can be used to replace or enhance organic detail.
Epoxy putty is stiffer than either solvent- or water-based putties, so it’s the perfect choice for adding new parts such as an extended radome on an aircraft or a turret bulge on a tank.
Wet a tool or toothpick to carve and shape the putty.
Sanding and painting
Once putty is dry, it can be sanded smooth. Sand with the shape to properly blend the filler and plastic. If shrinkage created a shortfall, add more putty, let it dry, and sand again. Wear a respirator and eye protection when sanding putty; the dust is an irritant.
In most cases, putty mimics styrene and disappears under paint. But large fills may absorb paint differently. I seal these areas with a little super glue, then sand them smooth.