The substance polyurethane is a broad name for a group of organic polymers produced from the combination of diisocyanate and polyols. Although many people don’t realize it, polyurethane is part of our everyday lives.
Polyurethane has applications in many household items, from foams to surface coatings to vanishing. This article will focus on its use as a surface coating.
Polyurethane is a popular surface coating solution for wood finishing and metals. It’s common practice to sand the surface of your finishing work for extra smoothness. However, do you need to sand the final coating of polyurethane?
Do You Need to Sand the Final Coating of Polyurethane?
Well, it depends. The general expectation is that you’ll have a smooth finish after applying polyurethane. In this case, there’ll be no need for sanding.
However, sometimes you can end up with a rough finish. Dust, bubbles, or tiny debris can get in the way. You’ll need sanding, in this case, to get rid of the obstructions, and produce an elegant, glossy finish.
So, suppose you have an imperfect polyurethane finish. How do you sand it?
Sanding involves the use of sandpaper to smoothen a surface. Sanding the final coat of polyurethane is a simple affair.
You can achieve it by following simple guidelines. Let’s first consider a few points.
How To Sand The Final Coat Of Polyurethane
Before you begin
- Firstly, there are two types of sanding – wet and dry sanding. Wet sanding combines wet and dry sandpaper. The fluids involved may be water or mineral spirits.
Dry sanding, on the other hand, employs one dry sandpaper. Using this method requires extra care because you can scratch or graze the final coat and spoil your smooth finish. Experts usually discourage the use of sandpaper with a rating higher than 600-grit.
- Another thing to note is the number of polyurethane coating required. Now, this depends on the type of polyurethane coat you’re using for your project.
For water-based polyurethane, you’ll need at least five coatings to get a good finish. On the other hand, you’ll need no more than four coatings for an oil-based polyurethane.
We’ll discuss both types later in the article, but first, let’s note the materials required to sand the final polyurethane coating.
You’ll need the following materials for this task:
- Sandpaper (600-grit for dry sanding, 400-grit for wet sanding)
- Water or mineral spirits
- Sanding sponge (for wet sanding)
- Vacuum cleaner
- Sanding block
- Tack cloth
We’ll now go over the sanding process for wet and dry methods.
Dry Sanding The Final Coat Of Polyurethane
The primary purpose of dry sanding the final coat of polyurethane is to eliminate all impediments to a perfect coating. Dry sanding involves a few simple processes.
Vacuum cleaning will eliminate dust particles on your coating. Using a vacuum cleaner, hover over every surface, nook, and cranny of your coating, so you don’t miss any spot.
Clean the surface:
This stage may be unnecessary if you did a good job with the vacuum cleaner. However, an extra layer of care isn’t out of place for assurance purposes.
Using a lint-free cloth soaked with either water or mineral spirits, wipe the surface of your coating to get rid of anything you might have missed during vacuum cleaning. Soak the cloth with water for a water-based polyurethane coating and with mineral spirits for an oil-based polyurethane coating. You can also use water for both coating types.
As a precaution, don’t use too much fluid for cleaning, so you don’t drench the wood. If the cleaning cloth is damp enough, then you’re good to go.
After cleaning, allow the surface to dry before inspecting it for dust, bumps, or other imperfection. If you still notice anything, it’s time to move to the next step.
Now is the time to use the sanding block. Place the 600-grit sandpaper on the block and gently swipe the areas that need smoothing. We can’t overemphasize the need to be gentle, so you don’t mess up your work.
For your health and safety, please wear a respirator when sanding. Wood dust can be detrimental to your health, and you don’t need health issues in the list of problems to solve.
Again, you’ll have to clean the surface but instead of a damp lint-free cloth, use a tack cloth this time.
You’d have a finer surface coating at this stage. However, if you’re unsatisfied or can still spot some imperfection, sand again, but use finer grit sandpaper.
We’ve gone over the dry sanding process. But, as simple and interesting as it appears, wet sanding presents a safer and more efficient method.
Wet Sanding The Final Coat Of Polyurethane
The wet sanding process also involves some vacuum action and surface cleaning. Remember to soak the cloth in water for a water-based polyurethane coating and mineral spirits for an oil-based polyurethane coating.
So, if you’ve vacuumed and cleaned the surface, and still desire a finer finish, take the next steps.
Pour some water or mineral spirits into a cup. Soak your sanding tool in the water or mineral spirit for a while. Some people recommend a few hours of soaking while others insist on 24 hours.
As a rule, the longer you soak the better result you’ll get. However, you shouldn’t soak for more than 24 hours. After that, all you need is a fair amount of moisture in your sanding tool.
It may seem like an unnecessary repetition, but again, use water for water-based polyurethane coating and mineral spirits for oil-based polyurethane coating. However, you can use water for an oil-based polyurethane coating and not the other way round.
After getting some moisture in your sanding tool following immersion, the sanding proper comes next. To do this, splash a little water on the surface and begin sanding circularly.
Intermittently wipe the surface with a clean to access your progress. Use a different cloth every time to have an accurate picture. A previously used material can transfer some mixture into the surface.
Continue sanding until you’ve gotten rid of the imperfections. Allow to dry for twenty-four hours, and then wipe the surface with a lint-free free cloth for a polished look.
In our explanations thus far, we’ve made distinctions between the requirements for water-based and oil-based polyurethane. It becomes imperative to consider these types of polyurethane coating.
As the name implies, water-based polyurethane has water as the base solvent for carrying the polyurethane solids.
On the other hand, Oil-based polyurethane employs organic solvents such as petroleum and minerals as the base solvent.
Although both water and oil-based polyurethane can form a strong and lasting protective film on your furniture, both types present some advantages and disadvantages.
Pros of Water-Based Polyurethane
Using water-based polyurethane comes with some advantages.
- Water-based polyurethane dries faster than its oil-based counterpart so that you can add more coats in less time. Essentially, it saves you time.
- Water-based polyurethanes are easy to remove. All you need is warm water.
- Water-based polyurethanes are eco-friendly. They give off little to no odor, making them more pleasant to use.
- Because they’re odorless, you can use water-based polyurethane indoors.
Cons of Water-Based Polyurethane
The following are the disadvantages of using water-based polyurethane.
- With water-based polyurethane, you’ll require more coating layers for a hard shell.
- Water-based polyurethanes are less durable against wear and tear than their oil-based counterparts.
Pros of Oil-Based Polyurethane
Although water-based polyurethane offers are a more environmentally friendly option, oil-based polyurethane is still popular in the market for various reasons.
- Oil-based polyurethanes produce a stronger and more durable shell than water-based polyurethanes.
- When using oil-based polyurethanes, you’ll need fewer coating layers for a smooth finish.
Cons of Oil-Based Polyurethane
Below are some disadvantages of using oil-based polyurethanes.
- Oil-based polyurethane takes longer to dry, so using them for finishing jobs will always be a time-consuming affair.
- Oil-based polyurethanes are not easy to remove. You’ll need mineral oils to get rid of them as they’re not soluble in water.
- Oil-based polyurethanes give off a pungent odor that takes time to fade off. The strong smell makes it very inconvenient to use indoors.
When to Use Water-Based Polyurethane
Water-based polyurethane is light, has a fast curing time, and doesn’t build on the surface. Therefore, it will be adequate for a project that requires a light and smooth finish.
When to Use Oil-Based Polyurethane
Oil-based polyurethane doesn’t cure quickly and forms buildup on the surface. However, oil-based polyurethane is scratch-resistant and suitable for a project where durability is the key. For example, you’d require oil-based polyurethane to polish hardwood floors, cabinets, and countertops.
Tip: If you’re working with oil-based polyurethane, use a nose mask to cushion the pungent smell from the coating.
You don’t need to sand the final polyurethane coat unless you find some impediments like dust nibs, debts, and bumps. But if you find them, you should sand the coat using the dry or wet method.
Wet sanding is the safer and more effective option. Wet and dry sanding involves nearly the same process except for water or fluid in the sanding process.
First, the sanding tool will require immersion in water or mineral spirit for a while. Secondly, you’ll splash some water on the surface before sanding.
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