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How to fill oxidised nail holes in floorboards?

Hi there,

I discovered that the subfloor in my new early 70's apartment is beautiful tasmanian oak, so I'm planning ripping up the carpet and sanding the boards back. However, in some areas the nails have oxidised, some to the point of some blackening around the nail hole and others it's just currently the nail. I obviously need to fill the holes, so I want to know if there is anything I can use that will also prevent the rust from continuing to seep through? Or is that wishul thinking...?!

Thanks

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Comments

MitchellMc

Hi @squeakysim,

You're right to be thinking about it now because the rust can keep bleeding through if it’s not dealt with properly. Before filling, it’s best to either pull out any rusty nails or, if they can’t be removed, treat them with a rust converter first to neutralise the oxidation. Once that’s done and they’re dry, you can fill the holes.

For filling, a timber filler designed for floorboards would be the way to go — something like Timbermate or similar, which is sandable and stainable. Just keep in mind that if the black staining around the holes is deep in the timber grain, it might still show through slightly after sanding and finishing.

Please let me know if you have any questions.

Mitchell

squeakysim

https://www.workshop.bunnings.com.au/discussion/comment/51442#Comment_51442

Thanks for that!

I’m excited that there is something out there that can help block it!

Yeah, the nails aren’t definitely not going anywhere!!

How does this stuff work? So it doesn’t just create a barrier, it actually neutralises the oxidation too? Will it seep into the wood and affect the nail deeper down?

I can imagine this being quite a tedious job for a large number of nails! How would you recommend doing the application? Just with a small paint brush? Is it likely to stain the wood? I’d like to keep the wood natural, so I don’t want to be staining it with anything darker.

Is there one brand that’s better to go for, if so why? Thanks.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/87868#Comment_87868

Hi @squeakysim,

You’ll definitely want to test it on a small, inconspicuous section of timber first to make sure there’s no discolouration or reaction with the wood. While most rust converters don’t stain timber significantly, it's better to be sure, especially since you want to keep the natural look.

I’d recommend applying it with a cotton earbud rather than a brush — just dip the tip in the solution and press it gently against the nail head. This gives you control and helps avoid getting it all over the surrounding timber. The product will convert the rust already on the surface of the nail into a more stable compound, which stops further rusting. It won’t seep deeper into the timber to treat rust down the nail shaft, but it does neutralise what’s visible and exposed.

Keep in mind it won’t block future staining completely, but it will stop the nail head from actively rusting further, which is usually the main culprit for those brown streaks.

As for brands, I’ve recently used the Bondall 250ml Ranex Rustbuster Rust Converter and found it worked really well. It’s easy to apply and does the job without fuss. So I’d say that’s a solid choice to go with.

Mitchell

squeakysim

https://www.workshop.bunnings.com.au/discussion/comment/109513#Comment_109513

Thanks for that!

Is this something you’ve used for this purpose? I’m trying to understand if the compound or whatever is leftover, is something that might absorb into the wood filler later and potentially colour it..? Or is it sealed in once it’s dried off?

Do I need to consider the type of wood filler too? I.e. not using water based in case it exacerbates any oxidation? Or once it’s dried this isn’t an issue?

Thanks!

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/122094#Comment_122094

A rust converter works by chemically reacting with the rust (iron oxide) on the surface of the metal and converting it into a stable, inert compound — usually a black or dark-coloured iron tannate or phosphate @squeakysim. This reaction halts further corrosion and effectively "locks" the rust in place, forming a sort of primer-like barrier. Once it’s fully dried and cured, it's sealed off and shouldn't leach or interact with surrounding materials, especially if you’re only applying a small amount directly to the nail heads. If you're concerned, you could then, with another earbud, apply a touch of PVA glue onto the head of the nails, sealing them further.

As for whether it would discolour the timber, in my experience, it’s unlikely to stain the wood if applied carefully. The risk is more about over-application or spillage, especially with light-coloured or untreated timber. That’s why testing on a discreet area is really important before doing all the nail heads.

In terms of using a filler afterwards, once the rust converter is completely dry and cured (usually after 24 hours), it shouldn’t affect the filler. You can safely use either a water-based filler at that point. I wouldn’t worry too much about a water-based filler triggering more rust, because the converter stabilises the surface and the water evaporates from the filler very quickly. Just make sure everything is dry and cured before applying anything on top.

Mitchell

squeakysim

https://www.workshop.bunnings.com.au/discussion/comment/130128#Comment_130128

That's great, thanks for that!

Is there anything you can suggest that is able to remove or lighten the stains in the wood already caused by the oxidation?

Cheers

MitchellMc

Hi @squeakysim,

Oxalic acid is often used to try to restore timber that has staining from oxidation. You could try Feast Watson WoodPrep, which is a product that’s designed to clean and revive timber. It contains oxalic acid and could help minimise the staining. Once again, I highly recommend testing a section first in an inconspicuous location.

Just follow the directions on the bottle. Typically, you’ll scrub it in with a stiff brush, leave it to sit for a while, then rinse it off. You might need a couple of applications for deeper stains.

Mitchell

squeakysim

https://www.workshop.bunnings.com.au/discussion/comment/51548#Comment_51548

Do you think there’s any chance that would react with the rust converter or counteract it?

Which order would you say is the best to go? The oxalic acid first then the converter maybe..?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/88352#Comment_88352

Hello @squeakysim

Using Feast Watson Wood Prep and a rust converter together can be a bit tricky, as they rely on different types of acids. Wood Prep contains oxalic acid, which is excellent for removing tannin stains and brightening timber, while most rust converters typically use tannic or phosphoric acid to neutralize rust and create a protective layer. These acids can sometimes counteract each other if not fully rinsed off, potentially reducing the effectiveness of the rust converter or leaving a residue that interferes with subsequent coatings.

To get the best results, it’s generally a good idea to use the Wood Prep first. This will remove any surface contaminants and brighten the timber, creating a clean base. Make sure to rinse it thoroughly and let the wood dry completely, as any remaining acid could weaken the rust converter’s bond or cause unexpected reactions.

Once the timber is dry, you can apply the rust converter to any metal components, ensuring they are also clean and free of oil or grease. If the rust converter leaves a residue, it’s a good idea to give it a quick wipe down before applying any final protective coatings to the wood or metal, ensuring a long-lasting finish.

If you need further assistance, please let us know.

Eric

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