Oak frames are popular for their strength, timeless look, and durability.
Whether it’s a garden pergola, a traditional gate, or a timber-framed structure, oak gives a solid, natural finish that lasts.
But when you’re working with oak, choosing the right fixings is essential.
Many people assume stainless steel bolts are always safe to use, only to find dark stains appearing around the bolt heads after a few months.
This isn’t rust.
It's a chemical reaction between oak and metal.
The culprit is the tannins in the oak itself.
Let’s look at what causes this and how to avoid it with the best possible fixings for oak frames.
Why Oak Reacts with Metal Fixings
Oak contains high levels of tannic acid, which can react with metals when moisture is present.
Even stainless steel can trigger this effect if there’s any trace of iron on the surface, from cutting, handling with carbon-steel tools, or even airborne steel dust from a workshop.
The result is dark or black streaks around bolts, screws, or washers, what’s known as tannin staining.
While harmless structurally, it can spoil the clean appearance of your frame. Fortunately, the right materials and preparation can prevent it.
Use A4 Stainless Steel Bolts for Oak
For oak frames, A4 (316) stainless steel is the best choice. It’s a marine-grade alloy that offers maximum resistance to both corrosion and tannic acid. A4 stainless bolts, screws, and washers are ideal for outdoor oak structures that are exposed to rain and humidity.
A2 (304) stainless steel fixings are also corrosion-resistant, but they’re slightly more reactive with tannins.
For interior projects or well-sheltered areas, A2 may be sufficient, but A4 gives you better long-term protection and helps maintain a clean finish.
At Fixabolt®, we stock both grades in a wide range of sizes, from M8 to M20 hex bolts, set screws, and coach bolts, all in durable A2 or A4 stainless steel.
Clean or Passivate Fixings Before Use
Even high-quality stainless fixings can carry microscopic iron particles from manufacturing or handling. These can spark the tannin reaction.
Before installation:
- Wipe each bolt or screw with methylated spirits to remove dirt and oils.
- If possible, acid-passivate the fixings. Passivation removes iron contamination and restores the natural protective layer on stainless steel. It’s a standard finishing process for marine and architectural work and can make a noticeable difference when fixing into hardwoods like oak.
Seal the Timber Around Fixing Points
The more moisture in the timber, the more likely tannins are to bleed. Applying a good quality wood oil, sealer, or exterior varnish around bolt holes creates a barrier that helps stop this.
Before inserting your bolts, brush a little sealer into each hole. After tightening, a small bead of clear oil or wax around the washer provides extra protection.
This step not only prevents staining but also helps reduce cracking and shrinkage as the oak naturally dries.
How to Remove Existing Tannin Stains
If dark marks have already appeared, don’t panic, they can usually be removed. The best method is to use oxalic acid crystals or a ready-mixed wood bleach designed for tannin stains.
To treat the area:
- Mix according to the product’s instructions.
- Apply with a soft brush and leave for 15–30 minutes.
- Rinse well with clean water and allow the oak to dry fully before applying oil or sealant.
The reaction will lighten or completely remove the black marks, restoring the natural colour of the wood.
When it comes to fixings for oak frames, stainless steel is still the best option, but it’s important to choose the right grade and take a few simple preventative steps.
Using A4 stainless bolts for oak, keeping the metal clean, and sealing the timber around your fixings will help you avoid tannin staining and keep your frame looking its best for years to come.
For builders, joiners, and anyone working with hardwoods, Fixabolt® supplies stainless steel bolts, screws, washers, and threaded rods designed to last, all at competitive trade prices with fast UK delivery.