It depends on what is on the redwood now. If a clear finish and it needs redoing then a light sanding, quick cleaning with a vacuum then apply a varnish.
If it is stained and you want a new color then stripping and sanding will be your best choice. Sanding alone could also work depending on what is on the redwood.
A finish coat(s) of varnish can be sprayed or brushed depending on the volume. I would spray with a HVLP or similar sprayer.