When you are painting an object, it is sometimes necessary to protect it from damage. If the object you are using will not be handled much, then you probably don't need to worry about it getting scuffed and starting to fade. On the other hand, if you are painting or staining a chair or other piece of furniture, then you need to think about what toll constant wear will take. Treating your furniture with a hardening agent will help it to wear better. In some cases, using a polyurethane product will work best, and in other cases a varnish will be better.
Polyurethane is a synthetic product made from a plastic suspended in a liquid. In most cases the plastic is suspended in water, but oil is also popular. Plastic will stand up well to most wear, but heat can affect it. In fact, setting a hot cup on a table treated with polyurethane can cause a white ring to form around the base of the cup. This ring will not buff out. Oil-based polyurethane will be tougher than water-based, but it too will have its limitations. Thus, if you have a piece of furniture that will only see light wear, then a water-based product is best, but if the furniture will see heavy wear then an oil-based product is better.
Varnish is not the same thing as as polyurethane. The main difference is that varnish will have a higher ratio of solids to liquids. In the end, this will make the coat made by the varnish much more durable. On the other hand, varnish will affect the color of the object you treat more than other options. Varnish is usually best for pieces of furniture that you will use outside. The ravages of mother nature take a much higher toll than the traffic that indoor furniture will see. Thus, you need a product that is up for the challenge.
Polyurethane and varnish are two hardening agents that you can use to treat furniture, but they are not the only options. You could also use a lacquer, or a product that contains acrylic. These products will also have their peculiarities that you should consider before you choose one for your project. In most cases, a good polyurethane sealer or a good varnish will give you the result that you want. To make sure that you get the result that you are after, it is necessary to talk to a painting expert like one from Klenosky Paint about your project and follow their recommendations.