Polyurethane resin or epoxy is a synthetic polymer used to make resins for plastic and rubber models. Polyurethane resin is coloured with liquid dyes, tinting pigments or agents and coloured powders. You can colour your polyurethane resin mix to make customised colours for your plastic models. Achieving the right colour for your resin will take a little bit of trial and error, as manufacturers provide no set amount or colour scale chart to guide you.
Set a thick layer of newspapers down on your work area and the floor around it to protect them from spills. Place the electronic kitchen scale on your work space. Turn it on and place a paper cup on it. Hit the "Zero" button to zero it out, or calibrate the scale so you will not add the weight of the cup to the weight of the resin when you pour it into the cup.
Label one cup A and one cup B so that you can keep track of the resins. Pour 28.4gr. of the "A" and "B" parts of the resin mix into their marked paper cups. Add 1 drop of your tinting agent or transparent dye to the "B" resin with the dropper or pipette. Pour the "A" resin into the pigmented "B" resin and mix with the wooden stick to make a 56.7gr batch of resin. Wait two minutes for the resin to harden.
Look at the colour of the hardened resin. Most pigments are highly concentrated, so you won't need much to colour your resin. Since there is no set rule or colour chart to follow, you must eyeball it and colour by hand. Transparent dyes will give a lighter, more delicate colour, so keep this in mind as you determine how much pigment you need to add to your resin. Keep measuring out 28.4gr. portions, adding a single drop more to the number of drops to each test batch until you achieve the colour you want. Write on the cup the number of drops of pigment or dye you used.
Mix up the amount of polyurethane you will need for your project. Weigh the mixed resin and add the number of drops of dye you need to get the colour you want. To determine the proper amount, multiply the number of drops by the number of ounces in the weight of the polyurethane.
Be careful when handling pigments and dyes intended for polyurethane resins as they are permanent dyes that are extremely difficult to get off of your skin or furniture.