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Sanding between varnish coats? Answered

Hi,

I've heard that sanding between varnish coats is critical, so that the next coat can properly stick to the previous one, is it that crucial?

What about the force you apply while sanding, how to know the sanding is effective, not too rough nor too gentle?

Thanks a lot for these great lessons.

Simon

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The idea behind sanding between coats is to ensure a smooth surface and give a great finish. Assuming you've sanded the base to a nice surface (around 800 grit, as a rough baseline), apply a coat of finish and let dry. Lightly sand again with 800 grit or higher to remove any debris that might have stuck to the finish while drying, then apply a second coat. Repeat as required.

IMHO, squared59 comes closest to the 'better answer'. Both types of surface usually respond to well to sanding between layers. Using 800 grit is going a bit too far for most items - generally 220 is as far as you need to go (exception: pens turned on a lathe) although I do go to 400 every now and then. I used shellac for most of my projects - quick and easy. Poly is excellent for finishes where you need water resistance. In every case, I sand between coats with maybe a final buffing with 4 ought steel wool and maybe paste wax.

Finishes are a complex issue and a whole class could be given on just that topic. There are 2 basic types of finishes. One creates layers like polyurethane. It is important to sand between each layer to create an mechanical bond between layers. The other are solvent based finishes like lacquer (not water based lacquers). Each successive layer dissolves and fuses to the previous layer so sanding between layers is not necessary.