Hi SteveH,
I like to keep things simple. This is my method. I don't claim it's the perfect way to go but it works for me.First job I do is gently attempt to give the handwheel a turn. If it turns and a needle is attached I check if the needle accurately passes through the hole in the needle plate. My reasoning is that if you get this far then chances are it's a machine worth working on.Next I remove needle, foot, needle plate and any bobbins and thread. Then I remove the machine head from its base and take off handcrank, stop motion screw assembly, handwheel and bobbin winder. I give the machine head and all the other metal pieces which are now detached a good spray with a penetrating oil (WD40) The wooden base or treadle table I coat with a good amount of water-soluble paint stripper. This will remove any varnish/paint/oil/dirt and can be washed away with soapy water without being too harsh on the wooden surface. I then let the wooden parts dry completely and then I firmly but gently give any external surfaces a thorough sanding being careful not to over do it. I then give all external wooden surfaces a good dose of wax furniture polish and then hand buff with a soft cloth. That's it for the wood - no more varnish! I like to see the grain of the wood and if a few dents are visible they give a little character. To clean non painted bare steel parts I use an abrasive cloth such as scotchbrite and very fine abrasive paper. To clean any painted (japanned) parts sewing machine oil and a soft cloth. Treadle ironwork I like to strip back and use a black aerosol spray paint such as hammerite. Hope this helps.