Replies

1. 
   flairwoodworks | Nov 10, 2008 07:55am | #1

   Larry,

   I think of a sanding sealer as a clear coat (often a washcoat of shellac) and primer as a coloured coat, used under paint.

   Chris @ www.flairwoodwork.spaces.live.com

    - Success is not the key to happines.  Happiness is the key to success.  If you love what you are doing, you will be successful. - Albert Schweitzer

2. 
   furndr | Nov 10, 2008 04:34pm | #2

   I was first introduced to sanding sealer by an old cabinet maker who also became my mentor. It is a clear coat and is used to "slick-up" cabinet-grade plywood after  it is stained. As the name implies it is designed to be sanded and contains stereates  to make it sand easy without clogging your sandpaper.

   You can begin spraying lacquer over stained plywood (or any other type of wood) without using sanding sealer, but the lacquer will raise the grain and then must be sanded, which is not easy. Sanding sealer also raises the grain but it is a breeze to sand. On porous wood I usually apply at least two coats of sanding sealer, sanding between coats. Then when you start applying the lacquer top-coat it doesn't penetrate as much and looks really good right off.  

    

1. 
   rabin | Nov 16, 2008 09:46am | #5

   Thanks for the the info

    

   Larry

3. 
   SteveSchoene | Nov 10, 2008 05:22pm | #3

   Primers for use under paint have a relatively large proportion of the binder and consequently less pigment.  This gives it very good adhesion to the wood.  Paint, with relatively more pigment, can bond well to the primer, so that the total system has better adhesion than if paint alone were use for the initial coat.  Some primers also have other beneficial properties such as stain blocking or imcreased resistance to water vapor transfer.  Using compatible primers can be a valuble step in achieving a good painted finish.

   Sanding sealer for clear finishes, on the other hand, is merely an expedient allowing easier sanding, and a relatively high solids content for quick build up of the film.  However, because they achieve these results with the addition of stearates (a chemical soap) the resulting finish is less durable than if the top coat had been applied without the sanding sealer.  In general they aren't needed and should be avoided unless commercial circumstances warrant the reduction in finish quality.

   There are exceptions--some top coats, such as some conversion varnishes, perform better over a specific sealer.  If so that product will be specifically called out in the manufacturers technical data sheet. 

   It should also be noted that Zinsser Seal Coat is not a "sanding sealer" as mentioned above.  It is simply dewaxed shellac, a material which does adhere well to most surfaces, and to which most all finishes will adhere. 

    

1. 
   rabin | Nov 16, 2008 09:40am | #4

   Thanks for the info

    

   Larry

2. 
   forestgirl | Nov 17, 2008 06:18am | #6

   Steve, we've picked out a new color for our house and painted a few boards that were part of a repair.  The label says the paint is "self priming."  What does that mean and how does a self-priming paint differ from not?forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 

1. 
   HowardAcheson | Nov 17, 2008 07:51pm | #7

   Jaime, "self priming" is primarily a marketing term. All paints are self priming in that they do not need a stand alone primer.Howie.........

1. 
   davcefai | Nov 17, 2008 08:49pm | #8

   I think you have to look at "primer" in a wider context than woodworking. Usually primers serve one or more of these purposes (and probably others:1. They adhere to the substrate wheras the topcoat may not do so well. For example fibreglass primer which allows you to paint fibreglass with oil paints and Calcium Zincate (I think) to prime galvanised iron.2. They protect the substrate. For example Red Lead primer in the good old days, rust primers.3. Others as mentioned above.When painting, a primer is often followed by undercoat before topcoating.