In order to obtain good adhesion or penetration, it is vital that the surface is clean, solid and free from loose wood fibres.
When the varnish has hardened completely. Normally 3 weeeks after application.
If you treat a surface, which has not been cleaned, there is a risk that the wood protection will not adhere properly to the surface and in turn it will peel off rather quickly. It is not enough just to brush the surface, before applying wood protection, removing the obvious problems such as cobwebs, dust etc. You need to clean the surface thoroughly and remove any kind of incrustations, so you are absolutely sure that the surface is completely clean. This also applies to new wood, which might have e.g. beginning fungal attacks after a long, cold winter in storage in an uninsulated room or even outdoors at the timber yard.
In general, you can say that a water based product obtains adhesion by attachment to the surface. Therefore, these products are more sensitive in terms of how clean the surface is. Alkyd based products have a better moistening/penetration ability. This means that solvent based products can adhere more easily to slightly dirty surfaces.