While not the longest lasting and will require at least 2 maintenance top coats every 6 months, (spring & fall),
Z-Spar Captain's Varnish is a very good beginners varnish. Easy to apply, relatively forgiving, and the finish can be prolonged, (as with any varnish), by keeping the varnished items covered and out of sight of UV rays.
The basics go like this:
Take the wood down to bare. 120 grit paper, followed by 220 and 320. Sand away all traces of old varnish and age stains/discolorations. Clean the bare wood with mineral spirits and allow it to dry over night.
The first coat of varnish needs to really penetrate the wood, so cut the varnish about 10% with mineral spirits. Allow it to soak in for a few minutes and repeat as necessary until the varnish stops soaking in and remains wet after 3 or 4 minutes. Wipe all the excess varnish away with a jersey rag dampened with mineral spirits.
Allow this initial varnishing to dry for at least 24 hours.
After drying, you're ready to start the build up coats. First, light sand, by hand, any "fuzz" from the surface. Wipe clean with a tack cloth or a rag dampened with mineral spirits.
Apply a full wet, but not soggy, coat of un-cut varnish and allow it to dry for about an hour, (called hot coating). Apply a second wet coat and allow it to dry for 24 hours. Continue this process until you have a minimum of 6 coats applied.
Allow the final coat to dry for 48 hours and thoroughly hand sand with 320 grit paper. Wipe away all dust with the tack cloth or a rag dampened with mineral spirits.
Do a final 2 coats, adding a small amount of
brushing liquid, no more than an hour apart, and your done with the base varnish.
Maintaining your varnish is critical so as not to have to go back through the whole previous process again.
Again, maintenance coats are at least a twice a year process, but go on quick and easy. One way to know when it's time to do maintenance is watch for the varnished item to slightly dull. Close inspection will reveal an almost light cobweb effect to the surface.
Maintenance coats are done by first scrubbing the varnished item with a fine refinishing pad, (3M white or equivalent), washing with soap and water, rinsing well, and allowing to completely dry.
Lay down a coat of varnish w/ brushing liquid, wait no more than an hour before laying down the second coat. You're done...
Pitfalls:
Trying to varnish in a dusty environment. Dust dropping in for a visit on fresh varnish is the number one biggest problem to overcome.
Trapped volatiles between coats is the second biggest problem. If you wait too long between hot coats the skin has formed on the previous coat and the next coat will not blend properly, causing the previous coat to wrinkle up. If you see this happening, immediately scrub all the new varnish off with the refinishing pad and mineral spirits. Start again...
Brushes:
Only use quality firm natural bristle brushes, badger hair is the preferred varnish brush. Clean the brush very thoroughly with mineral spirits, then wash with soap and warm water. Rinse completely, dip the brush in clean mineral spirits, tap out the excess, and store the brush in a tightly sealed zip lock bag. Make sure that the bristles are laying perfectly flat and not splayed out.
Do Not Work directly out the varnish can!
Pour the needed varnish into a clean, dust free container, and tightly reseal the can. Once opened, store the can in an upside down fashion. Do not pour any left over varnish back into the can. Throw it away...
Don't scrape the brush on the rim of the varnish cup, gently tap off the excess load, tapping the rim of the container just below the furrele.