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French Polishing
   What are the steps in French polishing?
  •  prepare the wood
  •  spit coating; (also called wash coating) optional, aimed at coating any purfling or inlay to stop colour 'bleeding' across to other surfaces during finishing. 3 coats.
  •  pore filling; (also called 'grain filling') aimed at creating a mirror flat surface on which to appy the shellac.
  •  apply shellac; (also called 'bodying') aimed at building up a coat of shellac thick enough to protect the wood. 6-8 bodying sessions.
  •  remove any oil; (also called 'clearing', 'stiffing' and 'spiriting off'.)
  •  rough levelling
  •  apply shellac; (also called 'bodying') second main application of shellac. 4-6 bodying sessions
  •  final levelling
  •  polish; (also called 'glazing')
   How do I prepare the wood? It will make French polishing much easier if the wood is well prepared. It should be clean, all old finish and dust removed. Sand smooth with successively finer papers. I generally finish with 1,000 wet and dry paper, used dry. After this, I burnish the surface with a piece of net curtain material or similar. When it is done it should litterally shine.
   How do I 'spit-coat'? The aim of spit coating is to place a little shellac on the wood for pore filling, and to cover any inlay or purfling, to prevent colour bleed. If you decide to fill pores using method 1 it could be ignored, except for 'spitting' any purfling or inlay.
  •   Spit-coating is done with a folded cloth, the shellac being 'wiped' on in straight lines. A piece of folded t-shirt material is ideal. 
  •   Saturate the cloth but use NO oil or alcohol. Re-fold to present a clean face to the wood after each pass. When each face is used, a new piece will be needed.
  •   Once the first spit coat is completed, it should be repeated twice more, with a gap of about 30 minutes between coats to allow the shellac to dry.

    

 Should I fill the pores? Pore filling is aimed at creating a mirror smooth surface on which to rub the shellac in French polishing. On large-pored wood like Honduran mahogany and walnut, the pores need filling. On small-pored wood like cherry and old Cuban mahogany, it may not be necessary. Spruce, cedar and maple are close-pored, and will NOT need proe filling. It also depends on the method of applying the shellac... it is less necessary with spraying and brushing, as the shellac can be applied much more thickly, and pores are thereby filled. On old mandolins it depends on how well the pores were originally filled, and how aggressively you sanded off the old finish. If you apply shellac when the pores should have been filled, you will know! You will still see the open pores after several coats. In the left photo, pores are open and visible, on the right closed.
   How do I fill pores?
  • Method 1: one of the easiest ways is by dampening a pad with alcohol and sprinkling on some pumice powder. This is rubbed over the surface in small circles. The dust this creates fills the pores. If the wood feels smooth but the pores don't look filled, and no pumice is left on the pad, add more and continue. Keep checking visually. If you sand too much, long ridges of pumice and wood dust will build up, and then must sanded away. This is done BEFORE any spit coating.
  • Method 2: (traditional French method) after coating the wood with a spit coat of shellac, saturate the muneca with alcohol, sprinkle a little pumice onto the pad and apply as you would the shellac, gradually covering the entire surface. The pumice thus fills the pores but leaves the surface very matt. With a clean muneca, once the pores seem filled, begin again with the shellac using a different pad. This is the method I use without too many problems.
  • There are other methods that vary a little, some more complex, which I have omitted as more suited to furniture polishing.
   Process of Applying Shellac? The process may look complex, but becomes second nature once you get used to it.
  • With a clean pad, add 2 lb cut shellac to the pad. Add about 12 drops onto the polishing surface. You will need less to re-charge the pad once it has been initially charged. 
  • Add a little alcohol to the pad, 6-8 drops. (Some expert say not to add ANY alcohol when bodying. Effectively you are thinning the shellac mixture, so it goes on thinner and takes more coats to build up any thickness. On the other hand, it goes on much more smoothly, and makes any levelling minimal. Your choice!!)
  • Add a drop of oil to lubricate the pad. (Without the oil the pad will stick) 
  • Pat the pad on the back of your hand to spread the oil evenly. 
  • Apply the pad to the surface to be polished, moving in small circles or figures of 8. Apply and remove the pad from the surface whilst it is moving to avoid it sticking and leaving marks.
  • As you rub, you should see a tell-tale opaque shellac 'cloud' following your pad. This quickly disappears as the alcohol evaporates. It is a good indication that you have the shellac-oil balance correct. 
  • As the pad dries, it will begin to be more difficult to move smoothly. Lift and add more alcohol and oil. Only add more shellac when you begin to lose your 'cloud'. 
  • Try not to re-cross recently shellaced areas, or you risk dragging partly set finish.
  • Once the area is covered using a rotating pattern, change to straight firm passes. This will help keep the surface smooth.
  • This is essentially the process.
  • Repeat this 'bodying' 2 or 3 times, then finish by 'stiffing'. Using added alcohol ONLY and pressing firmly, make straight passes back and forth over the entire surface bodied.
  • The instrument can be left to 'gas-out' for a couple of hours. This should allow any further oil to rise to the surface. If necessary repeat the 'stiffing'.
  • Allow to set overnight before any more bodying is done. If done too soon, the top will dry first, and will 'craze' as the deeper layers dry.
   What is 'clearing'? 'Clearing' (like 'stiffing') is the act of removing the oil used to lubricate the muneca. If you have not used much, or the stiffing went well, it may be un-necessary.
  • Using added alcohol ONLY, glide a folded pad on, and pressing firmly, make straight passes back and forth over the entire surface bodied. This should remove any excess oil. Some experts say that the shellac should be dried overnight before any 'clearing' is done.
  • Keep checking the surface of the pad, and refold periodically to a clean surface.
  • It may be necessary to 'body' and 'stiff' the instrument between 4 to 8 times before rough levelling.in order to build up a sufficient thickness of shellac.
   How long does shellac take to dry? In my experience, even in a humid environment, it is touch-dry in a matter of minutes, but takes a little longer to set properly. Once it is dry you can add more coats. If you apply several in a day, allow to set over-night before continuing. The thicker the coverage, the longer it will take to 'go off'.
   What is 'levelling'? This is the process of flattening out any imperfections, without burning through the shellac to the surface. Ridges and waves are usually symptomatic of uneven application of shellac. If you use a little alcohol with the shellac during 'bodying', you will find that this step becomes almost redundant.
  • For rough levelling, use a rubber or cork- sanding block and 400 grit wet and dry and oil as a lubricant. Some soak the paper before-hand to make the paper backing soft.
  • Wipe the area to be levelled with a light coat of oil, then sand with a circular motion until the area is totally smooth. On a bowl-back I tend to hold the paper in my hand.
  • Too much oil and you will glide over the surface and do nothing; too little and the sandpaper will load with shellac.
  • Wipe down the area to inspect your work, and when you are finished.
  • If you sand through the finish at some points, it can be re-covered in the next bodying session.
  • For final levelling, after another half a dozen or so bodying sessions, use a rubber or cork- sanding block and 800 grit wet and dry and oil as a lubricant.
   What is 'glazing'? Essentially it is polishing. Glazing is aimed at polishing out any micro-scratches left by the final levelling process. It is essentially the same as 'stiffing'. Some authorities suggest using a much lighter cut of shellac and applying that, but I have found using the 'bodying' muneca with alcohol only, works just as well for a mandolin, as enough shellac is left in the muneca. It is also possible to use automotive polish to lightly polish instead of the step. I have used Meguiars No 7 quite successfully.
  • Using the 'bodying' muneca, charged fully with alcohol, but not dripping. No oil!!
  • Work back and forth over the instrument in straight lines, with very firm pressure.
  • The harder the pressure, the harder the surface will be.

   A good article on French polishing furniture, another perspective: here

   How do I store shellac? Shellac should be stored in a glass jar, then you can see what is going on inside. DO NOT store in anything metal, it will react with it. It has a limited shelf life, though this depends on a number of factors.... temperature, humidity, strength, type used, etc. (I have kept and used some jars fopr over a year.)