Have you ever had your painted household items chip, peel, get dirty, or fade? If you have, you know how frustrating it can be to have to keep repainting the same thing year after year.
This is where a nice clear coat can really come in handy. A clear coat will protect the paint and be much easier to clean.
What is polyurethane?
Polyurethane is a polymer that is used in many different ways and to create many different things. Some polyurethane items you may recognize are skateboard wheels, foam sponges, spandex, gaskets, and hoses to name a few.
In this case though, we are talking about polyurethane as a clear surface coating or sealer.
Polyurethane is a plastic material and as such, when you apply a coat of liquid polyurethane to a surface, you will end up with a clear protective plastic film once it cures. It is durable, weather resistant, and corrosion resistant.
They even use it to coat bridges and roadways.
Can you apply polyurethane over paint?
You don’t necessarily need to paint a surface before protecting it with polyurethane, but if you would like a solid color, then painting is the way to go.
Before you start, see our tips on how to remove and prevent paint spills!
Polyurethane will adhere to and protect latex and acrylic paint as long as you have prepped the surface properly.
Will polyurethane stick to all surfaces?
Polyurethane will stick to most well prepared surfaces. The surface must be clean, dry, and free of dust, wax, and grease.
To prep your painted surface for polyurethane:
- Let your paint cure completely.
- Lightly sand the paint with 120 grit sandpaper to gently scuff and flatten the paint. Make sure to remove all dust before applying your finish.
- If you are re-finishing an item that has previously been waxed, you will need to remove the wax before applying your polyurethane. To do this, wipe the surface down with Mineral Spirits on a soft cloth.
- If there is any grease on the surface, be sure to clean it well.
What kind of polyurethane should you use?
The two main options for polyurethane are oil-based and water-based. There are some other options, like water-based oil-modified for example.
Oil-based polyurethane is a stronger finish than water-based and can hold up to heat better. However, oil-based will add a slight amber/yellow hue to the surface which can change the look of your paint beneath it.
Water-based poly dries clear and will not change the appearance of your paint color.
Additionally, water-based poly dries quicker, has less odor, and lower VOC’s (volatile organic compounds) than its oil-based counterpart, making it ideal for the home user.
Can polyurethane be used outdoors?
If you are planning to use polyurethane to protect outdoor furniture, make sure to check the label. Buy a polyurethane that is formulated for exterior use and is UV resistant.
Exterior polyurethanes are formulated in a way that allows the finish to expand and contract along with the wood during moisture and temperature changes throughout the year. Additionally the UV inhibitors will protect the finish from the harsh sun.
How to apply polyurethane over paint
First, you will need to decide where you will be doing the finishing process. You want a well ventilated area, with as little dust as possible.
Lay down drop cloths or newspaper (be aware that polyurethane can soak through newspaper, so pay attention to what surface you have underneath).
Polyurethane can be applied in several different ways. You can use a fine bristled brush, a spray can, a lint-free rag, a foam brush, or a roller.
Be sure to wear a respirator mask, like this one, and apply the finish in a well ventilated area, especially if you are using oil-based polyurethane or using a spray can.
For the best protection, apply two or more coats of finish.
How to get the smoothest and best looking results with polyurethane
While sanding between coats isn’t always 100% necessary for adhesion, it is a good idea to take this step. Lightly sanding between coats will knock down runs and brush strokes, remove dust that may have stuck to the wet surface, and lightly rough up the surface for the next layer to adhere to.
320 grit sandpaper is an ideal grit to use when sanding between coats. This will sand the surface thoroughly without putting deep scratches in the finish that coarser grits can easily cause.
Apply the final coat as smoothly as possible if you want a really flat finish.
How to achieve a glass-like finish with polyurethane
Wait for the final coat to cure, then sand with progressively higher grit sandpaper. Start at 600 grit and work your way up, being especially careful not to sand through the finish on the edges.
Automotive shops carry good quality high grit sandpaper that you may not find at the hardware store. One thing to note is that you will want to constantly check the surface of the sandpaper and remove dust and build up so that it doesn’t end up scratching your finish.
If your piece has detail work and is not a flat surface, you might want to avoid sanding too much so that you do not cut through the finish.
If you have a perfectly flat finish without dips or wood pores showing and you want a high quality mirror finish, proceed to buff with paste wax or car polish on a non-abrasive foam pad or microfiber cloth.
In the video up above, I show how I fine sand and polish a guitar that I built using these techniques. You can see at the end of the video how shiny and reflective the polished surface is. The labels on the bottles of polish are easily readable in the reflected surface.
How to clean up after you are done applying polyurethane
Good news if you decide to go with water-based polyurethane – you can clean up any messes with just soap and water.
Brushes can also be cleaned with soap and water if you are using water-based poly.
If you use oil-based poly, soak your brush in either mineral spirits or paint thinner to clean and refresh the brush for future use.
Applying polyurethane over paint is a great way to protect and enhance your furniture and other household items. It will protect the surface from scratches, prevent water and heat damage, be much easier to clean, and will also add a sheen or gloss to the surface.
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