To preserve the whiteness of wooden paneling made from spruce, poplar (aspen), or white pine, there are three key factors to consider: initial wood treatment, regular maintenance, and refreshing.

For the initial treatment, it's crucial to address raw wood promptly as it quickly oxidizes and changes color when exposed to air. Moreover, sunlight can cause yellowing over time due to UV rays. To prevent this yellowing process, it's important to use finishing products with UV protection. There are various finishing products available that contain dyes providing long-term UV protection: 

1. One option is to use an emulsified oil in a natural color. Look for a lightly white emulsified oil, like Prato-Verde's Brio Zero oil, that preserves the whiteness of the wood while protecting it from yellowing. These oils contain white pigments that offer UV protection. Conduct tests to gauge their effect on white woods.
2. Alternatively, you can choose a white oil specifically designed for paneling, such as regular Brio oil or Polyx-Oil Raw 3051. These oils whiten the wood, prevent yellowing, and provide UV protection. The advantage is that only one application is needed. There is a wide range of oils suitable for wall paneling and ceilings.
3. Another option is to apply a high-quality varnish with UV filters, such as a water-based SamaN varnish. Most floor varnishes have this feature.
4. While white acrylic paint offers protection, it masks the wood's interesting aspects. It's advisable to avoid using water-diluted white paint to achieve a white stain effect, as it can be challenging to control and correct in case of excessive overlap.

Regular maintenance involves cleaning the paneling regularly using a soft cloth and mild cleanser. This helps remove dirt and stains that could dull the whiteness of the wood. Avoid using harsh cleaning products that could damage the wood finish.

Restoration: Over time, the wood finish may lose its luster even with regular maintenance. If necessary, you can refresh the finish by lightly sanding the surface and applying a new coat of finish.

Ensure good ventilation to prevent moisture accumulation that could lead to stains or damage on your paneling.

Lastly, keep in mind that each type of wood has unique properties. It's always advisable to conduct a test on a less visible surface before treating the entire area to ensure the finishing product achieves the desired result.