Use a different type of applicator such as a pad painter or very short nap roller to spread the varnish. I have a problem with my exterior pine door. It was purchased new with no finish on it in 1989. Every year we sand lightly and apply another coat of varnish. Unfortunately the varnish keeps peeling down to the bare wood.
Peeling occurs because something under the paint film is preventing the finish coats from adhering.
This could be due to deteriorating undercoatings, deteriorating substrates or an incompatibility with something under the finish. Sanding sealers are not recommended for use outside. Because of the nature of clear finishes, some UV is able to penetrate through to the varnish. If there is a coating underneath that is UV sensitive it could be damaged and allow the top coat to peel off. Pigments in some sanding sealers can be moisture sensitive and attract damaging moisture.