Polyurethane is a polymer product that has many uses, including reproducing parts, making foam for seating and sealing wood surfaces. It is naturally either clear, white or yellowish in colour depending on the application and exact formulation. If you require polyurethane in a different colour, you must tint it. Tinting polyurethane is not difficult, but it must be done carefully as the tints involved are very potent.
Pour clear polyurethane into a container such as a clean paint can. Use more than you think you will need; matching the colour exactly between two different batches can be difficult.
Add a few drops of polyurethane tint into the container. Only use tint specifically designed for polyurethane to ensure that it does not cause unwanted reactions.
Mix the polyurethane and pigment thoroughly using a paint stick. Scrape the sides and bottom of the container several times as you stir to ensure that there are no pockets of unmixed material.
If you are using the polyurethane to seal wood, test the polyurethane by painting on a scrap of wood similar to the one that you plan to seal. If you are using the polyurethane to cast products in a mould, create a single item with this batch as a test.
Allow the polyurethane to harden.
Add more pigment, a few drops at a time, until you reach the desired colour. Mix again with the paint stick.
Test the polyurethane after every few drops of pigment. You can always add more, but the only way to reduce the colour intensity is to add more polyurethane to the batch. Keep careful notes on the amount of polyurethane and pigment used so you can recreate future batches if necessary.
Wear rubber gloves when working with polyurethane.