How to Use Wood Filler for Large Gaps

In a previous post here, we wrote about wood fillers, water-based, petroleum-based, and make-it-yourself.  We also discussed wood putty, epoxy wood filler, and plastic wood fillers.

It’s a good refresher course on wood fillers, and you might want to read it, or at least give it a quick review because we’ll discuss some of the same principles for filling holes in wood here.

Whether it’s filling nail holes or screw holes, or a knot hole, you’re likely in almost all of your home woodworking shop projects to have a need to fill a hole of some kind or fill a gap in a joint.

Let’s review the options once again as a quick reference.

Types of Wood Fillers

Water-based wood fillers:  A bit crumbly, but can be smoothed with a little water; cleans up easily; easily applied, and has a quick drying time.

Petroleum-based wood fillers:  Already smooth, stands up better to moisture and humidity.

Wood putty:  More flexible; stands up well to changes in both temperature and humidity; good choice for outdoor projects; not a good choice on raw wood, as it contains a chemical that can cause harm.

Epoxy fillers:  Comes in putty and liquid forms; quick-drying, sandable in as little as 15 minutes; fills holes, gaps, and cracks; a good choice for wood that has a little decay or has been damaged.

Powdered Fillers:  Good for wood surfaces, powdered fillers are comprised of sawdust and wood shavings.  When mixed with water, you’ll have a paste to apply to the surface.  It’s not structurally firm, and therefore not suitable for filling holes.  It can be sanded, finished with stain or paint, and improves the surface upon which it is applied.  It will cover and fill in scratches, too.

Home-made fillers:  A little Elmer’s glue and sawdust, mixed together well, and it’s ready to use; if you use sawdust from the wood being filled, the mixture blends in perfectly.

Most of these will have an application size of about ⅜ inch, but beyond that, hole or gap size will not have the best effect or longest life.

Using Wood Fillers

When it comes to using wood fillers, there are several factors to consider:

• Size. Nail hole, screw hole, knothole, scratch?  A larger gap?  Where is the gap – a joint?  A glue-up?  Crack in the wood?  Damage to the wood?  All of these will determine a good choice for which wood filler to use.
• Environment. Is the piece needing wood filler, an outdoor piece of furniture, or an indoor piece?  Is the location of the piece subject to environmental fluctuations in temperature or humidity? 

In applying these considerations, and the type of gap or hole being filled, to the list above, and the advantages/disadvantages noted for each type of wood filler will help you make an informed decision on which type to use.

You’ll notice, too, that Spackle is not on this list.  It’s a good product for filling holes and gaps in drywall, although it can be used on baseboards and trims.  Its gypsum constitution of calcium sulfate and glue dries hard and can be sanded and painted. 

But, most of us are more comfortable using a wood filler with actual wood (in the form of sawdust) to match the color of the wood being filled and take a stain to match.

How Big a Gap In Wood Can You Fill?

This is an easy question to answer if it’s just a nail or screw hole to be filled.

Water-based, petroleum-based, putty or homemade fillers will all do the job.  Whichever you have handy can be used.

But what of a larger hole or gap? 

To be honest, a homemade wood filler is one of the easiest, least expensive, and effective choices for even large holes.  You get to choose the sawdust and can match the wood you are applying the filler to, so color becomes a non-issue.  You can sand and finish with paint or stain, too, and it will match the wood it’s been applied to.

However, large holes and gaps can be best filled from an epoxy or acrylic caulk with silicone filler or a latex-based filler.  Powdered fillers can also be used effectively for large holes and gaps.

Each of them can be sanded, stained and painted, and are long-lasting.  They do not dry out quickly, dry hard, and can be drilled into for further joinery.

Wood putty, too, can be a good choice.  It dries hard, contains wood (so it can be sanded, stained, or painted), and does not shrink.

How large, though?  Beyond the ⅜ inch size mentioned earlier, but smaller than a well-measured and cut board replacement could cover.  Holes that go all the way through a board can even be filled with the right filler product – putty, homemade, for instance.

How To Apply Wood Filler

A few tools, a little patience, and some common sense govern the application of wood filler.

• Dry. Wait until the wood is dry.  If the wood has become discolored as a result of moisture, the filler will lock in the discoloration.  So, be patient and let it dry thoroughly.
• Putty knife. Use a putty knife to apply and push the filler into the hole or gap.  Apply a little pressure when doing so, and then use the knife to scrape away any excess.  Wipe any lingering filler away with a rag.
• Stain. If you intend to stain, use a stainable filler.  Epoxy wood fillers are not stainable, so avoid them; but, plastic (latex) and any filler with wood (sawdust, shavings) as a component will work as they are stainable.
• Patience again. Allow plenty of time for the chosen filler to dry.  Then, sand lightly by hand to smooth out the surface.  You might need to use a rasp or file if you’ve used an epoxy filler.  The product you’ve used will recommend a drying time and a curing time between 15 minutes to 24 hours, respectively. 

The size of the hole or gap being filled will affect drying time and the degree of patience you must show.  A nail or screw hole that requires little filler will dry more quickly and be ready for sanding in short order.  A larger hole or gap, though, will take much longer both to dry and cure.  Err on the side of caution when it comes to drying and curing time.

Some Common Wood Filler Products

In our earlier piece, we wrote about Bondo.  Bondo is an epoxy wood filler well known in woodworking corners.  It’s a good product to use in repairing and strengthening furniture that’s been damaged or has decayed over time.  It dries quickly and hard, will not shrink, and creates a very strong bond with the wood.

It’s also water-resistant and suitable for both indoor and outdoor pieces.  It can be painted or stained and is strong enough to take screws well for light load-bearing.

Another product to consider is DAP, a latex-based filler.  Plastic wood, so-called, is a high-quality material that creates a surface with the appearance of wood.  It dries hard, and in a shallow application, dries quickly.  Once dry, you can do anything to it you can do to wood – sand, cut, plane, nail into, screw into, paint, and stain.

It is strong and long-lasting but not as strong as an epoxy wood filler.  If you need a heavy load support, epoxy will serve you better than latex-based fillers.

Either of these two are good choices for larger holes and gaps for all of the reasons noted.  They are a bit of an overkill, though,  for simple nail and screw holes and small gaps and cracks.

What Fillers To Stock Your Shop With

As you’ve read, there are a variety of fillers on the market to choose from and homemade options.  We always say the right tool or, in this case, the right product for the right job.

To an extent, it depends on the projects you intend to take on in your shop.  If you’re new to woodworking, and the projects you will take on are simple, then maybe there will be only nail or screw holes to fill.  A homemade version would suit those purposes well, and you’re likely to have glue (maybe Elmer’s, for instance) in your shop already.  Your saw (any kind) will generate sawdust, and you should gather it up and save it for this purpose.

A water-based or petroleum-based filler will also serve the same purpose – simple nail or screw holes.  Each does a good job and are interchangeable for these simple purposes.

For more advanced projects, or repair jobs, where the holes and gaps could be larger, the epoxy and latex-based fillers are the better choice.  They’re stronger, harder, create a good bond with the wood, and can support some weight. 

Our advice is to keep a variety of fillers handy in your shop.  Be sure to check the shelf-life of each product you are considering, and anticipate the extent of your possible need based upon the projects you work on.  This will tell you the container size of each filler you are considering.

Glue belongs in every woodworking shop.  Woodworking shops generate sawdust.  These will give you a simple homemade solution.

Perhaps another simple solution from the list above is a good idea, too – – filler or putty.  Small holes are their strength and don’t use a lot of product to fill. 

Then, add to your list one of the extra-strength fillers, an epoxy or a latex-based product, for the larger holes and gaps you anticipate as possible needs.  We’ve given you their respective strengths of use, so choose accordingly.

There are lots of videos to choose from, and we watched a lot of them for you.  In the end, we turned to one of our reliable sources – This Old House.  Here’s a tutorial on all things wood fillers for you to watch and see some of what we have written about here.

Choosing the right wood filler will make the finished project better – in strength, in carrying capacity, in lifespan, and in appearance.  Give your project that well-finished look and display it with pride.