Sand and paint wooden stairs

Iso base important for white, because softwood, for example, resins and bleeds. Negligible for you. Primers are actually thinned varnishes. You can thin varnish and then have a primer. A thinned paint soaks into the substrate perfectly, gripping it. If you now paint over this thin layer with the same solvent, the varnish will adhere perfectly and also provide good coverage.

Thank you for the explanation. What about solvent-free paints then? Or should I rather avoid solvent-free ones because of impact resistance or something similar?

There is always some kind of solvent, otherwise it wouldn’t be liquid. Water, acrylic dispersion, or in the case of PU coatings probably nitro, alkyd resin, turpentine or something like that. For two-component K coatings, xylene. Once the solvent has evaporated, the coating is cured and no longer harms anyone. Until then, solvent sniffing may not be entirely healthy.

And yes, paints with traditional solvents, so-called oil paints based on turpentine or the more modern PU paints are more durable than water-based ones. Unfortunately, acrylates are not as impact-resistant.