Myths & Mysteries

[dunbarhamlin]

At least be realistic with your numbers - 1 thousandth of an inch is commonplace with half a thou closer to the normal limit

 

[woodbrains]

if we take 1 thou shavings with a very sharp plane, and overlap the strokes, the difference beween the high and low points will be 1 thou or less. This is less than the film thickness of most reactive finishes.
Mike.

Two points,

1. The "ridge" (ie the peak between overlapping planing furrow that's at risk of having the finish removed with subsequent sanding) is governed more by the camber of the plane iron than the thickness of the shaving.

2. Where did you get the information on finish film thickness?

Hello,

If the camber is across the full width of the blade, taking the full depth at the centre and tapering away to nothing somewhere just before the extreme edge and if the thickest part is set to 1 thou then the ridges cannot be higher than that. If you take a thou shaving with a radical camber, then the shaving will just be narrower, so the shaving thickness has everything to do with the ridge height. The plane stokes overlap so will mostly be less. The camber should only be slight on a smoother and possibly more on jacks etc. if you want that effect on these.

Film thickness, I cannot find a ref for it amongst my books, but I know I have come across it several times I'm sure you could find it on the web. A rule of thumb is, you only rub out a finish when the thickness is about 6 thou. Three coats of poly will be this thick or a touch more. Therefore minimum 2 thou per coat.

It wouldn't be a major problem cutting through the first coat by accident anyway, should you do so when removing nibs. Subsequent coats will cover any witness marks and will not need any where near the amount of sanding between coats, so should not be cut through again. If any one is too heavy handed and cuts trough on subsequent coats, it is not the fault of the initial prep, whether planed or sanded or whatever. Don't forget, it is easy to scrape out tearout deeper than a couple of thou and even easier to sand hollows with ROS or belt sanders, but no one worries about finishing in these instances. It seems harsh to pick on a finely tunes plane for causing problems with rub through.

I don't want to make an issue of all this, there is nothing wrong with sandpaper, but neither is there with polishing straight over finely planed surfaces. I'm not a fan of film forming finishes really, but they are a necessary evil in some instances. Too much sanding and too thick a finish all move away from the natural look of the wood which we all want, but we have to weigh up the pros and cons. I don't change my prep technique whether I wax or shellac or use a reactive finish and have never encountered any problems. If straight off the plane is good enough to take finish then I finish with whatever is appropriate.

Mike.

 

[Sgian Dubh]

2. Where did you get the information on finish film thickness?

Film thickness, I cannot find a ref for it amongst my books, but I know I have come across it several times I'm sure you could find it on the web. A rule of thumb is, you only rub out a finish when the thickness is about 6 thou. Three coats of poly will be this thick or a touch more. Therefore minimum 2 thou per coat. Mike.

Take a wood finish such as pre-catalysed lacquer as guidance for other film finishes. Manufacturers of these lacquers all recommend a wet film thickness of approximately 100 μm (microns); 100 μm is about the same thickness as a typical head hair. If you work in mils (1/1000'ths of an inch) all the polish manufacturers recommend a wet film of 4-5 mils. The other part of the equation is number of coats to apply, and with pre-cat this must be limited to three maximum at those wet film thicknesses. (This maximum wet film thickness is to prevent cracking or Chinese writing developing in the fully cured finish.)

The sums are easy after that. Take a typical pre-cat lacquer made up of solvents and 28% solids, although the solids percentage can vary according to the manufacturer and the grade or quality of the polish, but 28% is typical. Apply three wet coats of 100 μm each equalling 300 μm. 72% of the wet film will evaporate because it's solvent. This leaves behind a cured film of about 84 μm, approximately 3.3 mils (3.3 thou). To help visitors do conversions there are 25.4 μm per one mil (1 thou).

Apply the same sort of calculations to shellac or various types of varnish to estimate an approximate dry film thickness. Of course you also have to allow for how much you rub down between coats to get a general idea of dry film finish thickness. On the whole though, unless you are working with something like clear pour on epoxy finishes which can be very thick, most film forming finishes are likely to be somewhere between about 75 μm and perhaps 150 μm, or between about 3 mils and 6 mils. Slainte.