Stupid finishing question of the day....

[custard]

Which ingredient in French polish would you say is responsible for the mirror-like finish

It's not really an ingredient as such that's responsible for the mirror surface (yes the actual surface is shellac, but lots of other materials are also capable of delivering a mirror surface). It's more the process of french polishing. This process can transform the shellac into an absolutely flat surface; with the almost total removal of the tiny cracks, fissures, grain pockets, roughness, trapped hair or dust particles, brush marks, etc. All of which act to reduce the gloss level with many other finishes.

The purpose of the oil is to deliver the lubrication that allows the surface to be progressively worked to this level. What normally happens is that at some point the rubber grabs at the surface, some call this "burning", if and when it happens you then have to remove the burnt area by bodying over again. Now there are lots of ways of putting off this moment, adjusting the cut, re-charging the rubber, workshop temperature or humidity, the speed and pressure of your hand movements, etc, etc. When you've been french polishing for a long time you can almost feel when your rubber is about to grab, and you adjust these variables accordingly. Someone who is new to french polishing won't yet have that instinct, and so will quickly resort to using oil in order to keep going.

My issue with oil is that ultimately I'll want to remove it from the french polished surface in the final "spiriting off" stage. The more oil I've used along the way the more arduous that process becomes.

 

[worn thumbs]

I admit at the outset that I have never French polished anything.Come to that I don't have too much interest in the process as there are more durable alternatives available.I do remember that somebody in the past explained to me that sanding sealer is basically the ingredients that go into French polish plus a dollop of sodium stearate.Thats the stuff that is added to Frecut or Lubrisil paper to reduce clogging.

 

[Steve Maskery]

sodium stearate. That's the stuff that is added to Frecut or Lubrisil paper to reduce clogging.

Steve's Useless Piece Of Info For The Day:

Stearate (not sure if it is sodium, but certainly a stearate) is also used by magicians. When you see a bloke onstage snap his fingers to make a perfect fan of cards, it's because they are slicked with stearate powder.

 

[profchris]

It's the shellac which produces the French Polish shine. You can get close, as I do, by building up a coat of shellac and then levelling and polishing with increasingly fine compounds. No oil involved. But on a guitar, it's still a little less clash than proper French polishing.

As to suitability, there's nothing better sounding on a guitar than French Polish, even if it requires more babying than e.g. catalysed polyester. But not for kitchen worktops - different finishes for different uses.

 

[custard]

It's the shellac which produces the French Polish shine.

Except it's not!

Look at a piece of furniture that has both carved elements and casework elements. The casework is french polished, the carved work has the exact same shellac polish, but applied with a brush rather than a rubber. Both elements are coated in the same shellac...so do they have the same French Polish shine? No, they absolutely do not.

The shine is a function of the french polish technique rather than the shellac polish itself.

 

[Steve Maskery]

He's right, you know.

 

[profchris]

It's the shellac which produces the French Polish shine.

Except it's not!

Look at a piece of furniture that has both carved elements and casework elements. The casework is french polished, the carved work has the exact same shellac polish, but applied with a brush rather than a rubber. Both elements are coated in the same shellac...so do they have the same French Polish shine? No, they absolutely do not.

The shine is a function of the french polish technique rather than the shellac polish itself.

I wasn't clear enough - my point was that the shine comes from the shellac surface, not the oils or other ingredients sometimes used. But of course, it won't shine if the surface is uneven. If its flat enough to be polished then abrasive polishing can get a decent shine, though not the lustre of traditional French polishing.

 

[custard]

my point was that the shine comes from the shellac surface, not the oils or other ingredients sometimes used

Fair enough, I'd agree with that.

 

[johnnyb]

What allows shellac to get a gloss finish using a rubber is an odd semi soluble trait. Shellac slowly dissolves in alcohol.this allows an amount of burnishing to take place from the effects of the rubber. Anyone used pullover on cellulose lacquer? It's has a similar way of biting but not.
Oil is used when lashing a thick coat on to stop it sticking.
The semi soluble thing is obvious when dissolving buttons or cleaning out a spraygun after using shellac.
I usually try and avoid sanding sealer. It gives a softer finish film.(use lacquer coat on coat instead)