Using Sanding Sealer

Here's a question I received concerning myth #4 of our "Finishing Myths - Busted!" articles. 

Question: If sanding sealer weakens the bond of the finish to the wood, why use it?  Why not use the finish product itself.  Varnish on varnish, etc.?  As you mentioned, sanding sealer gives a smooth finish from which to start a finish, but if the bond is weak. (possible flaking or pealing) what's the advantage?   Or is the weaken bond minimal?

I am more confused than enlightened.

Help.

Here's my response:

Your question is a good one because when to use sanding sealer is a little confusing.  To be honest I never use it any more.

The main reason to use sanding sealer as mentioned in the the article, is because it's much easier to sand than varnish or lacquer.  Varnish and lacquer cure hard and can be difficult to sand level and smooth.  This is especially true for the initial "sealing" coat.  For this reason, sanding sealer can be used to provide a surface that is easily sanded level, which will make the following coats of finish go on smoother, which will require less sanding.

I haven't used sanding sealer for a long time.  Generally, if I'm using a finish such as varnish or lacquer and there's concern that it might not level out well, I simply thin it down.  By doing so the initial coat will go on better and require less sanding for a smooth surface.  Just be careful not to sand through the thin first coat.

As far as the possibility of the finish flaking or pealing off, I wouldn't worry too much about that.  The bond is weakened due to the mineral soap, but not to that extent.  Remember, the finishing myth that was busted is that finish doesn't "seal" the wood as well as sanding sealer.  This is simply not true.  The first coat of any finish will seal the wood.  So if you choose to use sanding sealer as your first coat that's fine, it will seal the wood, but not any better than the first coat of finish.

I hope this helps out.

-Craig Stevens