Hi @tsvn,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Restoring worn and aged timber can be an inherently damaging process, but due to the nature of your chairs, damage should be avoided as much as possible. So, the process should start with the least damaging methods, working back to the more destructive methods to absolutely minimise damage caused.
The first thing I would suggest is to give them a good cleaning with warm water and sugar soap to remove any surface-level dirt or grime. A non-scratch scourer pad will help to agitate and remove dirt and grime without scratching the timber.
After this initial clean, I'd suggest you use a stripper such as Intergrain Liquid 8 Timber Stripper alongside a plastic scraper to remove as much of the coating as possible. Ensure you wear chemical-resistant gloves such as these Sabco Professional Ultra Nitrile Chemical Resistant Gloves, as well as protective clothing and a mask. You'll also want to conduct this process in a well-ventilated area, such as a garage or outdoors, as the fumes can be powerful.
At this point, you should stop and assess the timber to see what you think. If the timber is back to bare, with minimal staining, another good clean may be the only thing you need to do before looking to recoat the timber with a varnish or oil.
If there are more deep-seated stains, you can use an oxalic acid-based cleaner, such as Diggers Rust And Stain Cleaner. Oxalic acid is a light bleaching agent that is commonly used in timber restorations like this to reduce and remove staining and restore aged timber. It should be mixed at a 1:10 ratio, with one part cleaner and 10 parts water, then applied to the whole surface and then scrubbed and allowed to soak for a few minutes before flushing with clean water. This will lighten the colour of the timber slightly, but when coated, the natural colour of the timber will be darkened and restored.
If there are still trouble areas at this point, you can sand them sparingly, but this should be avoided if at all possible.
Once the timber has been stripped back and stains removed, you can apply your choice of sealer, such as wax, oil or varnish, depending on the finish you are looking for.
With the timber restored, I would suggest you speak with an upholsterer for their assistance creating new cushions for your chairs.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
