Preparing and Sanding Wood for a Perfect Finish
Once the glue has set and your workpiece has really begun to take shape, it’s time to start the wood finishing process. This is the time to fill in any gaps and dents, and start sanding the wood surface down to a nice, smooth finish.
Learning how to sand wood by hand is an important part of the process. Getting it wrong can ruin a project at the last hurdle. Believe me, I’ve been there! Fortunately, lessons were learned and I’ve since collected several tips and tricks that’ll help you get the best results possible.
But, before we start sanding, you’ll need to learn the importance of filling wood holes and cracks for a tidy finish. So, in this chapter, we’ll take a look at everything to know about preparing and sanding your work to ensure a perfect finish.
What Is the Purpose of Finishing and Sanding Wood?
During the woodworking process, the wood surface is likely to take at least a little damage. A stray hammer strike can make dents, a slipped chisel or saw can leave a scratch, and sometimes the wood itself may chip or splinter, especially around knots. In other cases, some of your joints may have small gaps, or the router left a scorch mark.
During the process of ‘finishing’, you set about repairing all of these issues. In addition, you’ll make sure that any sharp edges are sanded down to make them safe to handle. Finally, when you sand wood, it enables finishes such as paint or stains to adhere better.
If you don’t prepare the wood surface and sand it down properly before applying a finish, all these defects will become even more obvious, leaving your project looking amateurish at best, and downright ugly in the worst cases!
Hand Tools for Finishing Wood
You’ll find a wide array of tools available for preparing wood to be painted or stained. The most common tool is of course sandpaper, which is available in different grits—more on that later. Another useful wood scraping tool for shaping and smoothing wood is a file for woodworking.
Along with rasps, which are much coarser and leave a rougher finish, woodworking files come in a variety of shapes, sizes, and teeth configurations that determine how coarse or smooth the finish will be. Typically, you might use a file or a rasp for shaping wood before the finishing process. However, a fine cabinetmaker’s wood file can help reduce the amount of sanding you need to do by removing the worst of the roughage.
This includes removing chipped wood fibers on the end grain, tool marks, and other rough surfaces. Another place a file can help is the inside edge of a hole. A file for wood can take care of these issues with relative ease, and normally much quicker than just sanding would.
How To Use a File for Finishing
We won’t go into too much detail here about using rasps and files as the majority of the work they do isn’t during the finishing process. Instead, they’re normally used for shaping wood, such as chair legs.
To use a file, hold the handle in your dominant hand. Use your other hand to guide the end of the file and apply pressure slightly. Files only cut on the forward stroke, so release the pressure as you pull it back to prevent clogging. For a smooth finish, scrape along the grain, rather than across it, which may leave deep scratches and kick up wood fibers.
Curved files are great for decorative moldings, while flat files are better for flat surfaces. When filing wood for finishing work, you’ll want a fine-toothed file for the best results.
How To Prepare the Surface of the Wood
Once you’ve scraped the worst of the damage away using a file, you can begin preparing the wood for sanding. Before you pick up a sheet of sandpaper and a sanding block though, you’ll need to fill in any holes or gaps, raise up dents, and patch up any larger cracks, splits, and holes that might be plaguing your project.
Different Types of Wood Fillers and How To Use Them
Wood filler has been helping woodworkers patch up their mistakes for centuries, if not longer! Nowadays, there are several types to choose from, ensuring there’s something for all your needs when it comes to your wood projects.
If you’re painting your project, you don’t need to be too picky about the type of filler you use. However, if you plan to use a clear coat of stain, you need to be a little more selective. Fortunately, fillers are available in all common shades, ensuring they won’t stand out like a sore thumb when stained.
Here are some of the most common fillers you’re likely to find.
Commercial Wood Filler (Stopper)
This is by far the most readily available type of filler and the best to use on unfinished work. Easy to use and quick to set, it’s handy to have a pot around the workshop. When choosing a wood filler, there are a few things to take into account:
- Interior or exterior use: The latter is more likely to perform better in extreme weather conditions, including rain, high temperatures, and frost. Meanwhile, the former is great for things that won’t leave the house, like a coffee table.
- Water-based or solvent-based: The former can be diluted with water for a wider range of applications, and also have a less potent odor. They generally dry faster too, in around 15 minutes typically. Solvent-based fillers will have a stronger smell and take up to an hour to dry normally, but they’re more resistant to water, humidity, extreme temperatures, and rot. They’re typically best for outdoor applications.
- Color: Nowadays, many fillers are stainable, just choose a color that is lighter than your planned stain or paint. If you’re using a clear coat to finish, choose a filler that best matches the wood and try to avoid using too much if possible.
- Consistency: Thicker fillers are best for plugging bigger voids, while thinner ones are best for filling in pores in open-grain wood.
To apply a wood filler, you’ll normally use a putty knife or wood scraper to press the filler into the gap or hole you wish to fill.
Two-Part Wood Fillers
Similar to the previous fillers in most regards, two-part fillers do however tend to set much, much harder than standard fillers. To apply, you first need to mix the filler with a small amount of hardener. This mixture is then pressed into place using a putty knife or scraper.
These fillers are great for rebuilding broken edges and larger holes. However, while they sand down well, they can’t normally be stained. So, while they’re a great choice for painted projects, they will typically show a bright spot when a clear stain is applied.
Wood putty is often mistaken for typical filler, but the two are pretty different. Almost always solvent-based, wood putty is best for finished workpieces, as the chemicals can react to the exposed wood of an unfinished project. Putty is often used to repair damaged window or door frames, but generally shouldn’t be used during the finishing process.
Sawdust and PVA
This classic method can still help fill small gaps in your joints. Simply mix fine sawdust with PVA glue, and press it into the gap using a putty knife. It’ll sand clean and if you use enough sawdust, it’ll be hard to see, though it can leave a bright spot when stained if too much glue is showing.
Wax sticks come in an assortment of colors to match the wood that you’re using. Most woodworkers apply wax in small wormholes or nail holes. It’s worth noting that most finishes won’t dry or set properly over wax, and wax sticks are normally used for projects that will have a wax finish applied or French polish.
In workpieces that have already been varnished or stained, you can safely fill small holes using wax. Just be sure to find the color that matches, or blend two colors together using a soldering iron to melt the wax, to get it exactly right.
Wax is often applied by simply rubbing the stick into the hole, but can be warmed up on a radiator first for easier application.
Filling Tips and Tricks
Filling wood cracks, holes, and gaps is relatively easy, but there are a few tips and tricks that can save you a lot of time later on when it comes to sanding everything down.
- Always work in a warm, well-lit environment: You need good light to detect all the blemishes and defects you’ll need to take care of. A warm workshop prevents the filler from drying too quickly and becoming crumbly.
- Ensure the wood surface is clean and dry: Again, this allows the filler to do its job properly. If there’s a lot of dust, the filler won’t adhere as well as it should. If the wood is wet, that can also prevent the filler from setting firmly.
- Fill deep holes in stages: Large holes should be done in two or three stages, allowing the filler to set between applications. This provides a stronger finish and ensures the filler seeps into all of the void.
- Press the filler into the hole or gap using a putty knife: Scrape away any excess filler from around the sides, but leave the repair slightly raised so that you can sand it flush later.
- Don’t rely on filler for large holes and splits: Large knot holes and other blemishes are best filled with solid wood or veneer rather than just filler, which may fall out later. This is especially important if you’re using a clear finish.
- Work systematically when filling: Go around your workpiece bit one side, one face, at a time. Fill the larger holes or gaps first to avoid running out halfway. If you’re using a two-part filler, take care not to mix too much at once. Allow the filler on the first face to dry before flipping your work around and working on the other side.
How To Patch Larger Holes and Splits
Some holes, often knot holes, are too big to fill just with filler as the filler is likely to fail. Not to mention it’ll leave a large bright spot, something you’ll want to avoid in work with a clear finish.
Instead, the best practice is to cut a diamond shape recess around the hole. Diamond shapes tend to blend in with the grain of the wood best and are fairly easy to cut compared to circles and squares. Here’s how it’s done:
- Find a piece of wood that closely matches the workpiece.
- Cut about a half-inch thick diamond-shaped patch out of it, large enough to cover the hole and a little more.
- Plane a small bevel on each bottom edge of the patch.
- Trace the outline of the patch onto your workpiece, ensuring the hole to be filled fits within it.
- Use a sharp chisel to chop out the diamond shape, tapering the recess to ensure a tight fit.
- Cover the bottom and edges of the patch with glue, place it in the recess, then cover it with a wider piece of scrap wood before hammering it down into place.
- Clean up the excess glue with a wet rag.
- When the glue has set, plane the block flush, ready to be sanded down. If there are any gaps, use filler to fill them up.
Patching up a large split or crack
Fixing a split or crack is done in the same kind of way. For splits that aren’t too wide, you can fill them with a matching veneer strip. You may need to use a dovetail saw to widen the gap a bit and allow the veneer strip to fit inside. Then, simply glue the veneer, and use a mallet to secure it in place. Once the glue is set, plane it flush, ready for sanding.
For larger cracks and splits, you can cut a strip of solid wood to fit inside. Like the diamond patch above, plane a small bevel along the bottom edges to ensure a tight fit. Then glue, hammer into place, and plane it down once the glue has set.
How To Raise Dents in Wood
It’s very easy to dent the surface of wood projects with a stray hammer blow, an over-tightened clamp, or simply by dropping it. However, shallow dents can be raised fairly easily if you prefer not to use filler.
Simply lay a damp cloth over the dent, then press the tip of a hot soldering iron to it. The steam that this generates is enough to ensure the wood fibers expand rapidly. Once you’ve finished, let the surface dry thoroughly before sanding flush.
The Importance of Sanding Wood
The final step before applying your finish is to sand your work down. This is the part where you’ll even out any steps on your joints, smooth out any excess dried filler, and take time sanding scratches out. Sanding wood smooth is an essential step, as it enables primers, paints, and stains to adhere to the surface properly.
When you don’t take time to sand wood, you might find you’ll have problems with peeling paint in the future or ugly bright spots on stained work.
Before we learn how to sand wood, let’s take a closer look at the different types of sandpaper you’ll need to use for the job.
Types of Sandpaper for Woodworking
Sandpaper is surprisingly diverse, with woodworking supply stores stocking many different types.
Types of abrasives
First of all, it’s worth noting that sandpaper isn’t actually made from sand. Instead, there are a few different abrasives, frequently known as grains or grits, used. The harder the material you want to sand, the harder the grit material needs to be. Here are the most common for woodwork:
- Aluminum oxide: This synthetic and remarkably durable grain is perhaps the most common and affordable type of abrasive used for sanding woodwork. It’s excellent for leaving hardwood surfaces smooth, and, being available in a wide variety of grits, it can also be used for other materials.
- Garnet: A natural crystal grain that is relatively soft, yet very sharp, making it ideal for woodworking applications. It’s generally used as fine grit sandpaper abrasive.
- Silicon Carbide: The most durable synthetic grain, it’s available in a wide range of grits, from super coarse to super fine. It can be used for both hardwoods and softwoods, as well as metals, plastics, and other materials. It also enables wet sanding.
- Alumina-Zirconia: Another synthetic grain, it’s very sharp and very coarse. It’s often used in metal work, but can also be used for the initial sanding of rough wood.
Sandpaper also differs by how coarse or fine it is. When shopping for sandpaper at the hardware store, you may have noticed different numbers on the back, such as 100-grit sandpaper or 220-grit sandpaper, for example. These numbers relate to the coarseness of the paper in question.
The higher grit sandpapers contain a larger amount of smaller grains compared to the lower grit sandpaper you’ll find. Many smaller grains produce a finer finish, while fewer larger grains are more coarse. Here’s an idea of what to look for.
- Extra Coarse: Typically between 26 to 36 grit, this is very tough and normally only used to remove old paint or other finishes. It’s not suitable for the finishing process, as it’ll leave deep scratches and eat through your wood in no time.
- Coarse: 40 to 50 grit sandpaper is usually considered coarse. It can be used to remove old finishes and also to shape wood, but shouldn’t be used for finishing a project.
- Medium: Typically running between 60 and 100 grit, sandpaper that is rated medium is normally best for rough cleaning and initial sanding of the wood.
- Fine: Ranging from 120 to 220 grit, fine grit sandpaper is what you’ll typically be using most of in the workshop. It’s ideal for hand sanding the wood down to a good finish, ready for painting or staining.
- Extra Fine: Anything from 240 to 400 is considered extra fine grit. It’s normally used between coats of paint or stain, while 600 grit, superfine sandpaper can be used for polishing jobs.
OPEN AND CLOSED COAT
Sandpaper can also be open or closed. Closed-coat sandpaper is densely packed with grains, covering more than 90% of the surface. Meanwhile, the grains on open-coat sandpaper generally cover between 50 and 70% of the surface.
Closed-coat sandpaper cuts much quicker than open-coat paper, due to the additional grains, or cutting edges, available. They also offer a more even finish for polishing jobs. On the other hand, open-coat sandpaper is less prone to clogging.
A brief note on sandpaper backing
In woodwork, we’re mostly concerned about the abrasive side of sandpaper. But, despite the name, not all sandpaper is backed by paper. Paper-backed sandpaper is the most affordable and most common, but you’ll also find a few alternatives:
- Sponge or foam: These sanding sponges are excellent for sanding contours and decorative moldings, as the paper shapes itself to the curves.
- Cloth/textile: Flexible yet tough, cloth-backed sandpaper is generally best for sanding power tools such as the belt sander or random orbital sander.
How to Sand Wood by Hand
So, now that you’ve picked up the best sandpaper for the job, let’s take a look at how to sand wood. Sanding wood by hand is surprisingly hard work, but it’s rewarding and a key skill to learn, since power tools can’t always get everywhere you need to sand.
Sanding by hand isn’t difficult, just a little time-consuming. That’s why many woodworkers use a power sander for the first round of sanding. When you do sand by hand though, here’s what to know
1. Always Sand Parallel to the Grain
Throughout the sanding process, sand parallel to the grain for best results. Sanding across the grain will take more wood off, but also causes visible scratches that may be difficult to remove.
2. Use a Sanding Block to Sand a Flat Surface Evenly
In its simplest form, a sanding block is a sandpaper-wrapped piece of scrap wood. However, you can buy cork or firm plastic foam ones from the hardware store. The best sanding block you can buy boasts a comfy grip and a clamp to hold pre-cut sheets of sandpaper tight. A sanding block not only leaves a more even finish but allows you to apply enough pressure to do the job quicker than just by hand.
3. Work From Coarser To Finer Grits
You’ll need to sand your workpiece several times, starting with rougher sandpaper, then using a finer grit to remove the scratches left by the previous round. For finishing work, assuming your wood is already planed relatively smooth, you’re unlikely to need anything coarser than 120-grit for the first round of sanding.
Of course, if you have raw wood that has a rough texture, or has been damaged by deep tool marks, you can use 100, or even 80-grit paper to quickly smooth it down. Either way, choose a good-quality garnet or aluminum-oxide paper for the best results.
This is the toughest round and the one that will take the most time. Keep sanding until all of the blemishes appear smooth and any filler sits flush with the surface of the wood. When you’re finished, dust your workpiece down thoroughly with a tack cloth to remove sawdust left from the first round of sanding. The fine sawdust left from the sanding process can damage your work if it’s not removed between sandings.
The next grit you use should be higher, around 150 or 180 to finish off any remaining blemishes. Go over the entire piece, and check that it’s all smooth, with no dips or bumps, before giving it another good dusting. Finally, using 220-grit paper, go over the entire workpiece one more time. Just quickly though, no more than about 30 to 60 seconds or so.
4. Raise the Grain
Raising the grain is the best way of ensuring a super smooth surface. After the last round of sanding with 220-grit paper, wipe your workpiece down with a damp cloth. Wait about 20 minutes. After that time, the tiny wood fibers that remain would have expanded, standing proud of the surface.
Use a fresh piece of 220-grit sandpaper, and go over the workpiece one last time with a light sanding. This will remove these tiny whiskers and leave a perfectly smooth finish. If you plan to apply a water-based finish, it’s particularly important to raise the grain before you finish sanding.
At this point, you’re ready to start applying your finish. But first, let’s take a look at a few handy tips and tricks.
Tips and Tricks for Sanding Wood
The following tips will help you ensure your work is the best it can be when you finish sanding.
- Take care with moldings: It’s all too easy to round off decorative moldings, ruining their appearance. Use sponge sanding pads to get the shape right, taking care to get right into tight spots.
- Sand down any sharp edges: If you haven’t put a chamfer on your edges, they’re likely to be pretty sharp. Use fine grit sandpaper to round off the edge.
- Sanding end grain: End grain fibers tend to grow in one particular direction. When sanding end grain, you should follow this direction for the best results. It’s easy to tell which way the fibers are going. Simply stroke it each way with your finger to find which way is smoothest.
- Use a putty knife for tight spots: For hard-to-reach areas, such as the corners of boards in a frame, this is a top tip. Simply glue a sheet of sandpaper to a flexible putty knife. Trim the paper flush, and use it as a makeshift scraper for wood.
- Use a soft, non-slip pad: You don’t want to spend time sanding your work down, only to flip it over to do the other side, and dent the work you’ve just done by placing it on an old workbench. A soft, non-slip pad for woodworking can be found online. Not only does it keep your work steady, but it prevents damaging your work as it lies on the bench.
Power Sanding Techniques
We won’t go into too much detail about power sanders here. But, if you do plan to use a power tool for sanding, I’d recommend a random orbital sander. A belt sander is just too rough for finishing work and can leave deep scratches.
Orbital sanders, however, do a good job of leaving just tiny scratches that are easily removed. They speed up the sanding process considerably, but you’ll still need to take care. Use fine sandpaper, and be sure to keep the machine moving evenly across your work, working in overlapping parallel lines. Use a shop vac to keep things clean.
If you apply too much pressure in one spot, there’s a very real chance that you’ll remove too much wood, leaving that section thinner than the rest of your work. Be aware that an orbital sander typically won’t get into tighter corners. Also, it cannot be used for decorative moldings, meaning hand sanding is still required. However, for things like doors, they’re a real-time-saver for taking care of the bulk of the work.
Painting and Staining Wood
With your workpiece sanded down and fully prepared, you’re ready for the final step of most wood projects; applying the finish. Sanding wood before staining, painting, or varnishing it is essential. But as you’ll see in the next chapter, you’re not quite done with sandpaper yet.
As the end approaches, be sure to check out the next chapter, where we’ll discuss everything you need to know about applying a variety of stains, paints, and other finishes.