Have you ever upcycled an old piece of wood furniture or made something new and realised that you needed to fill a few holes? You go to your DIY cupboard and realise you’re out of wood filler but there is a perfectly good tub of cheap, powdered, all-purpose filler right there. What do you do: go to the shop or just use the normal filler? What the heck is the point of wood filler anyways? If this is the dilemma you’re facing, this post is for you.
WHAT IS WOOD FILLER?
Wood filler is a material widely found in DIY stores. It is used to fill nooks and crannies in the wood when upcycling, making new projects or repairing old wooden items. It should be applied before any finish and once dry thoroughly sanded down. It is, therefore, suitable for bigger filling jobs. There are interior or exterior types of wood filler but each manufacturer should specify if there are limitations to where & how the filler can be used. This will usually be found somewhere on the packaging.
Wood filler can be found as a powder, similar to the all-purpose interior filler widely available that you mix with water, however, it’s more commonly found ready mixed in a tub or a tube. Additionally, it sometimes comes as a ‘2 part filler’ and you need to mix the 2 parts to form a useable paste.
The wood filler we find in shops today is made from some form of wood fibers that have been mixed with a bonding agent such as latex, epoxy and polyurethane. This is what makes the material dry very hard. So hard, in fact, that it can be a bit of a pain to sand afterwards. Once dry it will not contract or expand with the wood which can be a problem if you intend to fill exterior wood. However, using high-performance wood filler should help with this issue and using ‘2 part filler’ should completely remove the problem.
Wood filler usually comes in neutral colours which allows it to blend in with the original wooden piece.
Before this type of wood filler was available, a commonly used substitute to fill holes in wood was mixing sawdust with a bonding agent available to hand such as wood glue. This is still a great substitute for wood filler if you have the materials available but don’t have any wood filler to hand. Another reason to make up your own mix of wood filler is when you need to match the colour of the wood closely, for example, if you’re not planning to stain the wood and are unable to get wood filler in a matching colour.
WHAT IS ALL-PURPOSE FILLER?
All-purpose filler, commonly referred to as Polyfilla because of the very popular brand of filler, is a filler used for general interior repairs. If ever you needed to fill that pesky gap between your wall and skirting board or a small dent in the wall – this is the stuff you’d use. Similarly to wood filler, this stuff comes in a powdered form that you mix with water but you can also get ready mixed tubs and tubes. The big difference here is the composition of the filler. All-purpose filler is not made from wood fibers but rather other aggregates that are mixed with a bonding agent such as ethylene-vinyl acetate or a similar type of glue.
All-purpose filler can be found for interior and exterior projects so it’s important to check the packaging of these products. These fillers, once dry, will not contract or expand with materials they’re adhered to. The filler does harden as it dries and can therefore be sanded.
These types of fillers usually come in white but there are a few other options available such as grey or brick coloured for filling exterior render or brickwork.
WOOD FILLER VS ALL-PURPOSE FILLER: CAN YOU SUBSTITUTE THEM?
Ok so this all sounds great, but can you use all-purpose filler on wood or not?
Yes, you can use all-purpose filler on wood, however, there are some circumstances where these two shouldn’t be substituted.
One is when filling an outdoor wood project. All-purpose filler, the interior kind, is not waterproof so exposing it to the elements outside will destroy your repair over time. Moreover, as the filler gets wet it may rot your piece of wood before it falls out. Note that not all wood filler is suitable for outdoor use either so you need to be reading the filler packet to make sure it’s suitable to be used in a way that works with your project.
The second circumstance when not to substitute is when filling big gaps. This is because wood shrinks and expands as the temperature and moisture levels in the air change – all-purpose filler does not. As a result of this movement, the filler will crack and eventually fall out. The majority of wood fillers also don’t shrink and expand, however, certain brands and types are much more resistant to cracking than all-purpose fillers. 2-part wood fillers are the best when it comes to moving with the wood to avoid cracking.
Another problem with substituting the two is the colour. Remember that all-purpose filler is white whereas wood filler colours vary. This means that when you stain or dye the project, the white patch of filler will likely stand out more than coloured wood filler would. I went against my rule of using it on outdoor projects and used it on the herringbone tabletop I made because it’s kept inside a gazebo away from direct rain. It looks great but if you look closely you can tell where the filler is. It’s not as clean of a finish as I’d have liked. If you’re using solid wood paint on your project then this shouldn’t be a problem.
One final problem, which applies to both wood and all-purpose fillers, is that once you’ve sanded them, they become smooth (duh!). This does make them stand out on wood that has a really obvious grain texture running through it, especially if you’re staining or applying dye instead of a solid colour.
To summarise, use wood filler when:
- Working on an outdoor project
- Filling a big area
- Using a translucent finish such as stain or dye
WOOD FILLER FAQS
Now that you know whether wood filler is the right material for you, here are some tips on how to actually use it in the most effective way.
How long does wood filler take to dry?
I reviewed several popular wood fillers and found that drying time on the packaging ranges from 30 mins to 2 hours. In addition, you need to consider how big the hole you are filling is. For example, if you are just filling over a screw head, this will take significantly less time to dry than if you are filling in a 10cm gauge in a piece of wood. As a general rule of thumb, refer to the manufacturer’s instructions on your filler tub and add a little bit extra for bigger filling jobs.
Should wood filler be primed before applying a finish?
No, you don’t need to prime wood filler before applying a finish. However, priming is a good idea when you’re using wood paint over low quality filler as it will likely have poorer adherence properties. This could cause the paint to peel or chip easily from the filled area in the future.
If you are staining the wood or using wood dye, you do not need to prime the wood filler because you need the surface to be porous to soak the stain in.
How does wood filler look when painted or stained?
Unfortunately, the answer to this question really is: it depends. And it depends on many factors such as the type and quality of wood filler used, the type and quality of paint, stain or dye used, the type of wood you’re working with, and any previous finishes applied to the wood.
To attempt to answer this question, I ran a small experiment with some materials I had lying around.
Experiment materials
- x3 pieces of scrap wood
- All-purpose interior filler
- Ronseal multi-purpose wood filler in ‘natural’
- Rust-Oleum chalky finish furniture paint in ‘antique white’
- Ronseal 10 year wood stain in ‘natural oak’
- Wilko traditional interior wood dye in ‘deep mahogany’
To run the experiment, I firstly drilled some holes into both sides of my scrap wood. I then went ahead and used my all-purpose filler to fill holes on all x3 wood pieces on the top end. I used wood filler on the bottom ends. Once the filler dried, I gave the wood a good sand.
I then applied the finishes – one per plank. Once the first coat was dry, I applied a second coat to the dye and stain finishes. The chalk paint did not require a second coat. Below are images of the experiment – the finishes are wood dye on the left, chalk paint in the centre and wood stain on the right. Wood filler filled holes are at the top of the planks.
The result is certainly interesting. The first thing we see is that before any finishes are applied, the wood filler blends in much better than the all-purpose filler. However, applying a clear finish would not fool anyone, especially on the middle plank which is a little darker than the other two. I think in part it stands out so much on the other two pieces due to my very basic application and only a basic sand. I think if the intention was to leave these pieces unfinished, much more care would need to be taken to ensure the best possible blend. However, regardless of how much it is sanded, the lack of wood grain in the filler will still be obvious.
Looking at the finishes, the chalk paint covered both fillers perfectly on the middle plank. If your finish is a solid colour, you don’t need to worry about your filler standing out and you can safely substitute expensive wood filler with some cheaper all-purpose filler, provided your job does not meet any of the ‘use wood filler’ criteria above.
Now for the wood dye. The wood dye has been applied to the left-hand plank. It’s immediately noticeable that on the first coat, the dye is much brighter in the spots where both types of filler sit. It is especially noticeable in the area with all-purpose filler. After the second coat, the filler has blended a bit more over the wood filler however it is still a bit brighter than the tone of the dye on the rest of the wood. The second coat did not improve the dye sat on top of the all-purpose filler at all. It took on a purplish hue which is completely different to the rest of the wood. Additionally, and this isn’t noticeable in the pictures, after using the wood dye on the all-purpose filler it went a bit ‘bitsy’. It was almost like the dye was too wet for the filler and it was threatening to dissolve it as I was applying it.
The stain fared a little better than the dye. Unfortunately, however, both types of filler were still visible after applying the finish. I think a good sand and a third coat may have blended it in much better. Generally, there isn’t a noticeable difference between wood and all-purpose fillers.
CONCLUSION
If you’re going to apply a solid colour wood paint as a finish, it doesn’t matter which type of filler you use as both, wood and all-purpose interior filler should cover nicely. If you’re planning on using traditional wood dye, avoid using all-purpose filler as it becomes a little too discoloured and wet after application. With wood stain, either type of filler works. Make sure to take your time preparing the wood as any little imperfections will show up in the stained finish.
