First of all. finsihing should not be considered as an afterthought. It has as much to do with the final appearance as the wood itself. I have seen poor quality woods made to look good with good finishing techniques, and I have seen good wood and woodworking techniques obliterated by poor quality finish.
Without proper preparation, the actual finishing material can not do its job. To look its best, wood MUST be sanded prior to any finish application, even if the wood was carefully machined. Proper sanding involves more consideration than what we have time for here, but, besides grit, you must also consider the direction of the sanding, whether it is orbital or in line sanding, and whether the sander has padding or not.
STANDARD VARNISH- is a tried and true finish that’s been in use for about 100 years. Be sure to allow adequate drying time. The best finish work is either rubbed out or polished. There just is no way to get a dust free perfect finish that doesn’t require rubbing. For the woodies I use a varient of standard varnish called spar varnish. Spar varnish is formulated to be somewhat soft so that it can expand and contract with temperature extremes, and has a U-V inhibitor to help protect the wood from sun damage. Even so, clear finished wood that is constantly exposed to the weather will need consant maintainance, regardless of the finish.
POLYURETHANE-gives similar results to standard varnish. It’s one advantage is that it is hard, making it desirable for use where there will be abrasion. However that same hardness makes it brittle, and somewhat prone to cracking under impact or in high stress situations. Also, applied improperly, there can be failure to adhere.
LAQUER-is best applied with spray equipment, though very small projects can be done with a brush. Nitrocellulose laquer has been around for a long time and is realtively easy to use. It is a fairly weak finish that is primarily used on furniture. Modern technology has made numerous formulations of specialized laquers, each with it’s strengths and weaknesses.