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Sanding just before applying top coat

Gordon Fowler Member

I am looking for references for the reasons behind the the period between sanding and applying a top coat.

3/2/22       #2: Sanding just before applying top co ...
Daniel Shafner  Member

You can useb me as a reference. The reason is that a fresh tooth (scratch pattern) gives you your best shot for your finish to adhere (grip itself) to the substrate, assuming that your sanding is effective, and that your scratch pattern has the proper tooth, and that you did not instead of good sanding, burnish (polish) the substrate with a grit that is too fine (not coarse enough) for your finish to get its good grip.

To sand in too far advance of finishing, the scratch pattern can or might close up.

The game in finishing is adhesion to the substrate, as well as intercoat adhesion.

3/3/22       #3: Sanding just before applying top co ...
Chemmy  Member

Mr. Shafner is spot on, when it comes to almost any finish that does not reliquify itself to a degree, as the Evaporative finishes such as nitro lacquer, solvent acrylics, shellac, etc. Do.
Since you did not state what finishes your using, my only comment is if your using evaporative finishes, the only reason you sand between any coats is to both level out any non flat areas that may have been missed, or if the first coat, any fine raised grain, or to remove anything that either landed in the finish or was sprayed over unintentionally. For any you can use any grit paper that will not cut through the first coat, or proceeding coats.
3/3/22       #4: Sanding just before applying top co ...
James Sunshine

I have known many who sand Friday leave for the weekend come in Monday and do a wipe down and proceed with another coat to have it not adhere. What was the idea of saving time created a mess.

3/3/22       #5: Sanding just before applying top co ...
Gordon Fowler

The question is not why sand, but the role that time has on the grain raised by sanding. I teach finishing and I have found that when you combine the "why" of things with the "how" of things people tend to develop better habits. There is excellent literature documenting finish chemistry, light optics, surface tension and adhesion; I would like to be able to add similar documentation on what happens to grain raised by sanding and time intervals.

3/4/22       #7: Sanding just before applying top co ...
Chemmy  Member

Hi Gorden, if your concern is mainly grain raising and the sanding of the raised edge fibers of the tubelear pores, that when after wetted stand above the surface of the surrounding wood substrate, that is one thing. But you seem to be asking something I don't quite understand?
Dry sanding of wood does not raise any grain, but the grain itself will raise if left uncoated, especially in a humid atmosphere, even in areas like Arizona, though not as quickly or as much. This is the reason why you should coat the wood as soon after sanding as possible if clearing or priming the surfaces.
When staining, dyeing, or the use of any solvents or mediums that contain water or alcohols especially, which is most of the finishing coatings out their now days, grain raising will always occur.
In past times shellac was used to both raise, seal and stiffen those fibers so they could be sanded flush to the surrounding surface and then coated with an oil varnish, which for discussions sake had virtually nothing in it that would cause the grain to raise again.
That concept of wetting, raising, and stiffening of those fibers has and still is the main way of taking care of the problem, with low solids materials being used.
If you looking for more in-depth information, just e mail me and we can discuss this to your hearts content.

3/5/22       #8: Sanding just before applying top co ...

For arguments sake let's not talk about wood/grain.

If you painting a surface(metal) abrading the surfaces with a fine grit like 150-240 massively increases the surface area. It also provides a uniform smooth surfaces.

Once you spray there is a green(gel) time for most products. It could be 5 minutes or 5 hours, every product is different. During the green time you will get chemical bonding between the coats.

If it goes to cure stage you are now back to a mechanical bond. Between coats you once again want to sand to increase the surface area, remove defects, and most importantly remove the sheen. Most of the coatings we use are plastics and do not like to stick to very smooth surfaces(600grit and above). 320 grit is the typical recommendation between coats. You can see 240 grit scratches thru the different coats.

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