If you’ve ever unboxed a model horse only to find it oddly tacky or blemished with tiny bubbles, you’re not alone. This report explores the root causes behind sticky surfaces, bubbles, warping, and other casting flaws—so you can understand what’s going wrong and how to avoid it.
Whether you’re new to polyurethane casting or an experienced collector, our goal is to demystify these problems, bust myths, and offer practical, science-based solutions. From mixing ratios and exothermic reactions to mold materials and humidity control, we’ll walk through how to achieve consistent, long-lasting results.
For collectors, it’s about more than first impressions—it’s about how a model holds up over time. Solid casting may look great at first, but it can conceal long-term risks. Knowing what to look for can help you avoid surprises down the road.
Let’s dive in.
Resin Stability Once Cured
Polyurethane resins like Smooth-Cast 300 are stable when properly mixed and cured. Under normal conditions—away from UV, heat extremes, and harsh chemicals—they can last for decades. Problems only arise when environmental factors or improper techniques compromise the curing process.
Common Misconceptions
The claim that “resin can go sticky at any time” is false—if the resin was mixed and cured correctly. Once cured, Smooth-Cast 300 becomes a strong, inert plastic. It won’t suddenly become tacky unless exposed to degrading environmental conditions or flawed from the start.
What Causes Sticky Resin?
Inadequate Mixing: Poor blending of resin and hardener leads to uncured, tacky spots.
Wrong Mix Ratio: Too much or too little hardener can prevent full curing.
Expired Resin: Old resin may not cure correctly, even if mixed well.
Environmental Degradation: Prolonged UV, heat, or humidity can break down cured resin over time.
Solid Castings and Exothermic Reactions
Resins cure through an exothermic reaction that releases heat. In solid pours, especially over 4 fl oz, this heat can become trapped, causing “runaway” reactions.
Risks Include:
Sticky outer layers with cured cores.
Warping, cracks, or surface bubbling.
Long-term degradation despite initial appearance.
How to Prevent It:
Stage Pouring: Pour in layers and allow cooling between pours.
Small Batches: Mix smaller quantities to reduce heat buildup.
Why Solid Casting is Risky for Collectors
While solid casting may seem sturdy, it’s more prone to hidden flaws, heat stress, and eventual warping or brittleness—especially when exposed to seasonal changes. It’s also heavier, more expensive, and harder to repair if dropped.
Better Options: Rotocasting or staged pours reduce heat and bubble formation, improving long-term durability and paintability.
The Pressure Pot Myth
Pressure pots compress air bubbles during curing but:
Don’t eliminate bubbles—just shrink them temporarily.
Don’t control internal heat buildup.
Can result in bubble re-expansion if resin isn’t fully cured before depressurizing.
Rotocasting: A Better Method
Rotocasting coats the mold’s interior with resin, using less volume and distributing heat more evenly.
Benefits:
Less Heat: Thin walls allow heat to dissipate.
Fewer Flaws: Reduces warping, cracks, and bubbling.
Lighter Models: Easier to handle and less prone to damage.
Cost Efficient: Uses less material without sacrificing durability.
Humidity and Resin Curing
Humidity can affect curing in several ways:
High Humidity:
Triggers surface condensation → bubbles or stickiness.
Alters hardener chemistry.
Causes yellowing over time.
Low Humidity:
Speeds up curing → surface cracks or incomplete reactions.
Tips:
Keep humidity stable with a dehumidifier or humidifier.
Avoid casting in extreme moisture or dryness.
Refrigerate resin/hardener for more working time and less bubble risk.
Choosing the Right Mold Material
Smooth-On Mold Max (e.g., Mold Max 30) works well with Smooth-Cast 300:
Heat Resistant: Manages exothermic reactions.
Flexible and Durable: Releases cleanly and captures fine detail.
Non-Stick: Less chance of residue or surface defects.
Best Practices:
Fully cure molds before casting.
Use mold release for intricate shapes or high-detail areas.
Air Entrapment and How to Fix It
Causes:
Fast mixing.
Narrow mold channels.
Resin too thin to force out trapped air.
Solutions:
Vibration Plates: Dislodge bubbles in small parts.
Pneumatic Vibrators: Ideal for large-scale or full-body molds.
Vacuum Degassing: Pulls air out of mixed resin.
Slower Mixing & Pouring: Reduces air inclusion.
Smart Mold Design: Include venting and sprues for airflow.
The Smooth-Cast 300 Issue (Early 2000s)
What Happened:
Some batches failed to cure due to contamination or improper raw material handling.
How It Was Fixed:
Smooth-On improved batch testing, reformulated mixes, and recalled bad batches.
They’ve since maintained high reliability and strong customer support.
Note: Our studio never used affected batches.
Collector’s Quick Guide: How to Choose a Quality Resin Model
✅ Ask whether the model was rotocast or poured in layers.
✅ Avoid solid casts over 4 oz unless verified clean, cooled for multiple pours, and bubble-free.
✅ Look for models made using vibration, vacuum, or degassing.
✅ Be cautious of models that arrive hot or heavy.
✅ Ensure the model was cleaned and fully cured before shipping.
Final Thoughts
We stand behind every model we cast. If there’s ever a problem, we’ll make it right—like the time I forgot to vibrate a solid head for the Rotocast Elayna, which caused subsurface pinholes. We replaced it at no charge.
The secret to happy customers? Ship only fully cured, clean models that need little to no prep. It takes extra time, but it’s the right way to do things.
About the Author
Larry Backstrom has been casting model horses since 1999, partnering with his wife, sculptor Debbi LerMond of HorseModels.com. With thousands of casts under his belt, Larry blends traditional craftsmanship with modern tech—running a professional 3D printing lab using Formlabs and Phrozen gear, alongside tools like GOM, Blender, and Lychee.
Whether it’s resin or 3D printed models, his focus is always on quality that lasts.
One Response
Thanks, Larry for putting together such an informative post regarding model horse resin casting. There are so many nuances that you need to navigate through to get the perfect castings. I hope this helps anyone who is either casting their own editions or looking to get into it as a business. 😉