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How to use a wood filler
Buy or make an appropriate filler.
Stainable wood fillers are designed to be stained so you can match them to the rest of the wood. Some water-based wood fillers are also stainable, but they are specifically meant to be used indoors.
You can also make your own wood filler by adding sawdust to carpenter's glue to form a paste. Another option is epoxy designed for wood, which is hard-wearing and can be carved.