Introduction to Resin Casting

by Albert Lane, Rio Grande Hobbies Co.

(Editor's Note: Albert Lane is the owner of Rio Grande Hobbies Co. and manufacturer of On30 resin kits. Albert has been kind enough to write up this introduction to resin casting. I've added some pictures of Albert's On30 Porter Coal Tender kit so that you can see some of the results of his methods.)


"I hope that you will find the fun and adventure in casting your own parts from resin that I have. It is a simple process and with insight into a few pitfalls to avoid it is a lot of fun. Let's get started!

The first step is to decide right now if you are doing just a few personal castings or want to go commercial and do bunches of castings. You need to know so that you don't waste money on stuff that will not get used. The most important thing to remember about resin is SHELF LIFE, SHELF LIFE, SHELF LIFE!!

If you buy a gallon of resin good chance is that if you don't use it in 4 or 5 months it will go bad. Buy what you will use. I will be talking about Smooth-on products because I have been using them for years and know the good and bad side of them. I just bought a gallon of resin on ebay and will let you know what I think of it as I use it.

AeroMarine Products ebay Store

Smooth-On website

You need a mold to pour your resin into, and something you want to mold. Make your first mold as simple as possible! A one piece mold with no undercuts will get you up and going fast. So here we go!

I glue my plug to a smooth surface with either rubber cement or glue. Rubber cement is easier to clean off any excess that smushes out when you press it down to the surface. You dont want the mold rubber getting under your plug! Build a box around your plug and be sure the joints are filled so the rubber won't leak out.

Decide what kind of mold release you will be using. I started with the silicone mold release and still use it. Draw back is the moldings need to be scrubbed clean before use or glue and paint will not stick! There are paint friendly releases on the market now but once you use the silicone release you can't change over to the other in your molds.

You now have what is called THE MASTER. Be sure to save this for future molds because molds have a way of breaking at the most inopportune time!! Spray a light dusting of mold release into the master and let it dry before making a mold.


How many copies are you going to make? I do a quick and dirty (Q&D) on scratch built items I want several of,ie, a truck sideframe. This way you can cast up several to make a mold that when cast will produce several complete units at once. The Q&D method works well if you are going to cast only a few of your plug and Hobby Lobby has a water base mold material that will work fine for this method. I use Oomoo RTV from smooth-on for my molds. I suggest you stay away from latex mold materials. I use the Oomoo 25 because it sets up faster.Just like the resin, the RTV molding compound that is 1:1 mix is by far the easiest and most reliable way to go.

Get a big box of popsicle sticks from hobby lobby to use for mixing all your stuff, some medical cups with the hash marks (drug stores is a good source for a few), small paper dixie cups about 1 oz. When I mix my mold material I like old frosting cups. They are flex plastic and the rubber doesn't stick to them. I pour the rubber into the paper cups (part a in one and part b in the other) then into the the frosting cup for mixing. The a & b parts are most often 2 different colors so it is easy to see when you have it well mixed. Be sure to scrape the bottom to get a good mix all the way thru.

Pour the mold material into the master, not on the plug, but around it.Try to work out any air bubbles that form on the plug. An old, small paint brush works well for this. You now want to work the air bubbles out of the RTV. A vacuum set up would be great, but thats $$$. I started by softly thumping the sides of the mold box with my fingers until the mold set up. This is a DRAG!! I now use an aquarium air pump from Wal Mart (6.00) that I glued a box to the top. It has a small vibration that gets most air bubbles out. If you use a slow setting RTV (like 24 hours) it will get all bubbles out. I'm impatient so I go with the 3 or 4 hour kind.

Use the RTV traces in the frosting cup as a guide to how your mold is setting up. When you can roll it in your fingers and it doesnt stick to you or itself the mold should be almost cured. Remove the box sides (peel off) If they stick to the mold wait longer. Then gently work the mold loose from the base and the plug, working from the outside in. You should have a nice clean mold. If any RTV got under the plug it will leave little pieces of mold at the openning. Trim this carefully off the mold. Let the mold set and dry some more without the plug in it.


Lightly spray the mold with mold release. A heavy coat will cause a bad casting as it messes with the resin. I have lost molds because of using too much mold release. Just a good dusting works well. If you see release agent in a shinny film/puddle on the mold you used too much. Using the medicine cups, measure out a & b resin and pour into a dixie cup. Stir well and pour into the mold. Look for tiny air bubbles trapped in the mold and work them free. A tooth pick will work well here.

I use a piece of .010 or .015 clear plastic sheet for a smooth finish on the bottom on the casting. This saves tons of time in cleaning up your castin. Holding the plastic in both hands place one edge at a side of the mold and lay it across the mold in a rolling kind of movement. This will push the excess resin in the direction you are moving. If you end up with an air pocket or bubbles you don't want just lift it off ,add more resin, and reapply. The plastic will peel off the casting when the resin hardens and can be used over and over. I use clear so I can see what is going on under the plastic. When the resin sets remove the plastic and gently peel the mold sides away from the casting. The casting should almost pop out of the mold.

These are the basics and a good point to start."

Editor's Note: If you have any questions for Albert, you can email them to him by clicking here.

As Albert receives questions he will send me the answers and I will post them here.

My thanks to Albert for taking the time to write this article and share his knowledge with us. Be sure to visit Albert's ebay store at: Rio Grande Hobbies

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This page updated Feb 26 2007
Webpage © Lawrence Rickert
Title and contents © Albert Lane