It's almost impossible to explain what I'm doing. There are three reasons.
First, I've been studying stains, finishes, and solvents for at least four years now, and I am still just a student. I will be a student for years.
Second, there's just no substitute for experience. Technique plays a BIG role in results. That means someone interested just has to get out and try it, and there's no absolute right way.
Third, I gave my word to keep some of what I've learned off the net. There are people who make a living using some of the restoration techniques I was taught, and my teacher, a person who once made a living restoring antique furniture, insisted that I respect that.
I was blessed to build a house for a fellow a few years ago who was a retired paint chemist ( he was 82 at the time ) and I pestered him incessantly about varnish chemistry, lacquers, shellac's, and oils. He also built and finished his own furniture and was a great resource for a wood finish junkie. The problem I have now is I can't remember who taught me what.
I'll explain what I think I'm at liberty to discuss.
The first decision is when to refinish, touchup, or just leave it alone. For me, I try to guess what will have the best result in preserving the wood for posterity. It's a personal decision. I haven't lived a few hundred years and been able to observe wood over that period of time, so I can only guess.
As far as finish removal and replacement goes, the first step is to ID the finish. Most air rifles have lacquer finishes, but there are different types of lacquer. They can be acrylic, nitrocellulose, or catellized lacquers. I had to learn how to ID the material. I did that by getting samples and studying them, and it's take a while to write about it.
Restoration experts resort to chemical strippers as a last resort. I'm afraid I can't go any farther than that. There is an enormous amount of BS on the net that is contrary to what I've just said.
Dent and scratch repair. The best thing is to start with a stock that doesn't have any broken wood fibre. I use water and heat on beech, but you have to be very careful for changes in wood color. I never use steam. I don't want to alter the stress in the wood and have it change shape. The rest would take too long to explain, but if you go slowly and carefully study what's going on I think anyone can arrive at their own techniques.
Dealing with the darkening of wood that has been exposed to daylight and air for years is an area I'm just starting to understand. I use a couple techniques to lighten it or "camoflage it" in the new finish. I don't think I'm strong enough here to offer anything illuminating.
I use oil based dye concentrate because it's as close to the factory original process as I can get. There's a discussion over on Airgunadvice.net that covers it a little more. I thin the concentrate about 3 parts dye to 7 parts solvent and spray apply it. I find it more useful than anilene dyes because it will partially dissolve into the sealer. This allows me to thin the tone of the color in areas that may have broad age darkening so they don't show up in the restored piece. I do this by using 0000 steel wood on the cured sealer. It works for me.
The finish is just the appropriate spray applied lacquer, which has been leveled up and buffed with a paper towell. Lacquers have limitations for application which are common knowledge, but if you've never used them it's helpful to practise until you see a fault, such as blushing, appear so you have some idea of what your environmental limits are.
Well, I'm tired of typing and I've just brushed the surface of what this beginner has learned.