Hi @adelemarkwell,
A warm welcome to the Bunnings Workshop community, and thank you for your question.
You'll have to seal the timber in some way. There are a few options depending on how the timber looks and the finish you are looking to achieve.
The first option is to repaint them. This is the best option if you remove the paint and find that the timber beneath doesn't look quite as nice as you'd hoped. There is a great deal of customisability with paint as they can be tinted to a huge variety of colours. Doors and window frames will typically use a semi-gloss or gloss paint such as this Dulux Aquanamel Semi Gloss Interior Exterior Paint.
If you find the timber is very nice, then you could use an oil or varnish to protect the timber and enhance its natural aesthetic.
Oil will give the most natural look and feel to the timber as it penetrates the grain of the timber, maintaining the natural feel of the timber. If you'd like to use an oil, you can use either decking oil, or exterior specific timber oils such as this Intergrain 4L Natural Exterior Nature's Timber Oil. The downside to using oil is that it will require the most upkeep of the three options. Maintenance coats will need to be applied once every 1-2 years, depending on the wear.
The final option I could advise is varnish. Varnish is a clear polyurethane coating that seals the timber, but being clear, it maintains the natural look of the timber. Whereas oil will penetrate the grain, varnish will create a film-like seal on the outside of the timber, meaning you will lose the natural feel. Varnish will require less upkeep than oil but will not last as long as paint. If you'd like to use varnish, look for one that is UV resistant and specifically states it is for exterior use. This Bondall 1L Satin Monocel Gold Marine Clear Timber Varnish is a good option.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
