If you do a little research you will find many arguments with regard to the best finish to use on a guitar. However, most will agree that whatever finish is used, as thin a finish as possible is best for tone and clarity.
Nitrocellulose lacquer was most commonly used and is widely considered one of the best finishes. It is however solvent based, explosive when sprayed, bad for the environment and very bad for the user. Newer factory finishes include two-pack plastic or epoxy finishes but all can be toxic and require special spray facilities to apply.
I prefer to use natural, renewable, non-toxic substances and for this reason I tend to avoid lacquers, acrylics and polyesters. The backs and sides of my instruments are finished as standard with an Oil varnish containing mainly tung oil. I use either a French Polish or the Oil varnish finish on softwood soundboards.
My oil finish is taken from a method used by Kevin Aram. If it is good enough for a builder of his renown it is good enough for me! This finish imparts a satin look to the instrument and is very minimalistic allowing the natural grain and figure of the wood to shine through. I love it. I much prefer to see the natural beauty of the wood rather than a plastic gloss coating.
French Polish has long been regarded the finish of choice for fine guitars. It has stood the test of time being centuries old, and is an organic finish derived from the secretions of the Lac beetle. It is perhaps the most labour intensive method of them all. I don’t choose it for ease! For this reason it is an optional finish for the whole instrument rather than my standard. French polishing can take me up to 6 weeks, sometimes more.
The drawbacks of these finishes are that they can be more prone to marking and scratching than modern polymers. However I'm creating instruments for people who love guitars, and who will take a little extra care with them. I also offer a clear plastic pickguard to protect the soundboard if required.
I am currently experimenting with some of the more recent waterborne lacquers that show promise. These are non-toxic and may be offered in the future as one of my finishes.
I haven't found a satisfactory method of pore filling yet and it is the subject of frequent discussion among guitar builders. Many methods have been tried. Some are more succesful than others. Eventually, over time, the finish will sink into the pores and the pores will begin to show no matter what you do. I have given up trying to hide them. I actually prefer to see the beauty of the wood as nature intended and so most of my finishes are “open pore”.