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What is sanding sealer?

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  • What is sanding sealer?

    After reading this message board and SSW for a while a have a faint idea what sanding sealer is used for. However, I haven't seen anything like it for sale in here. Hence my question:

    What is sanding sealer really? Is it something I could mix myself? Or what could I use instead of it?

    I also wondered, can it be used before stain too or only with paint or varnish.


  • #2
    Allow me to quote from the Book written by Bob Flexner, one of the respected names in finishing in North America.
    "Separate sealers exist not to seal the wood but to make the first coat of finish easier to sand. ... the first coat will lock some wood fibers in an upright position. These fibers make the surface feel rough. ...Some finishes are difficult to sand, however, so manufactures make up special products to use as the first coat. Thees products are Sanding Sealers."
    " Sanding sealers are the regular finish with mineral soap (zinc stearate) added.....similar to hand soap. It acts as a lubricant to help keep the sanding paper from clogging up....."

    The article goes on to explain that each basic finish like varnish, water base and traditional lacquers each have a 'sanding sealer' that makes a sanding barrier coat, and has adhesion properties to match the rest of the finish. However, a sanding sealer is NOT a primer coat like used with paints. The Mineral soaps will weaken the adhesion of the top coats to the wood itself.

    "Polyurethane doesn't bond will to finishes that contain mineral soaps or shellac that contains wax, so you shouldn't use a separate sanding sealer or wax-containing shellac under polyurethane."

    One of the most common home brew sanding sealer is a 1-lb cut of de-waxed shellac. The second is, of course, a very thinned out version of the finish product you are going to use. My guess is that many hobbyist have moved on to the water rag to "raise the grain", sand lightly, very thin coat of finish, sand again (and loose some of the finish in the sanding) then apply the 1st top coat (of many coats.) The point here is that sandpaper is cheap to a hobbyist, to a factory with spray equipment, the sanding sealer is cheaper.

    A word in your ear if I may:

    If you are going to use wood dye, or other colorant, with Pine or other softwood, using a sanding sealer first will help prevent the "splotchy" look of the pine wood. (Splotchy is a technical term that is defined only by experience of using wood dye without a sanding sealer on Pine.)


    * * Flexner, Bob: Understanding Wood Finishing 1994, Rodale Press, Inc Emmaus, PA USA (Revised editions of this book are available at many book stores)


    • #3
      And just to further mix you up:

      A sanding sealer is also sometimes referred to as a washcoat

      DW788. -Have fun in the shop or it isn't a hobby anymore.

      NOTE: No trees were killed in the sending of this message, but a large number of electrons were terribly inconvenienced.


      • #4
        Sanding sealers

        A "wash coat" can be any "coating" that has been thinned out, it is used to partially seal the wood.

        There, are some finishers who do not use any sanding sealers, they use "wash coats" of the same clear coating that they use for their final clear coating.

        Today, vinly sealers are very popular, they have excellent resistant to mositure, so, they are very popular with kitchen cabinet and bathroom vanity manufacturers.

        Years ago, most sanding sealers were made up of a "shellac mixing lacquer," it was the sealer used for nitrocellulose lacquers, they are still sold today they do not contain stearates..


        • #5
          Minwax makes a sanding sealer. I'm not sure if you can get that in Finland or not?
          Here's Minwax's definition of their sanding sealer:
          A clear-finish primer formulated for application over bare wood. It provides a finish that can be sanded to create a smooth surface under oil-based polyurethane topcoats like Minwax® Fast-Drying Polyurethane or Minwax® Super Fast-Drying Polyurethane for Floors. Its quick dry feature allows for sealing and topcoating your project in one day.



          • #6
            Sanding sealer under Polyurethane

            Normally, sanding sealers or shellac are not recommend to be used under polyurethanes.

            Usually, the label will mention this information, and then state that polyurethane is self sealing and needs no sealer.

            This "new" sanding sealer, maybe a reduced polyurethane which many finishers normally do on their own.

            But then again Minwax may have developed another product. I really don't know for sure.


            • #7
              Originally posted by Mac
              But then again Minwax may have developed another product. I really don't know for sure.
              Where there is a market, there is always room for a new product.

              If I only had, ideas, products and money to develop them then I would be......
              wait if I had money I wouldn't need the rest...never mind.
              CAЯL HIRD-RUTTEЯ
              "proud member of the best scroll sawing forum on the net."
              Ryobi SC180VS scroll saw EX21


              • #8
                No, Minwax polyurethane sanding sealer.

                I just read the Minwax product page, and they do not have a polyurethane sanding sealer.

                Usually, the poly is thinned down 50 % or more, and this acts as the sealer, it is allowed to dry, it is then sanded, and then the poly coats are applied.

                Some, finishers prefer thinning all their poly coats.


                • #9
                  Mac I don't believe Theresa said the sealer was polyurethane. She said they had a Sanding sealer . As a matter of fact she took the quote from this page.
                  CAЯL HIRD-RUTTEЯ
                  "proud member of the best scroll sawing forum on the net."
                  Ryobi SC180VS scroll saw EX21


                  • #10
                    They do have a poly sanding sealer.


                    On the url that you listed, they do show a sealer for poly, but it was not listed on their page with their poly products, thats why, I did not find it.

                    I have used their poly many times, I never knew, that they made a poly sanding sealer, I always, thinned it out for sealing and for my final clear coats.

                    Mac S


                    • #11
                      Well, all I know is that Minwax is a brand that I know, trust, and use ~ so if their instructions say it can be used with poly, then I will use it with poly!! That's why I do read the manufactures instructions on any product I use.

                      And Satu - I use the sanding sealer under stain without any problems.





                      • #12
                        Oh dear!

                        I read the whole thread of the answers and now I'm even more confused. It seems to be much more complicated than I thought. So let me rephrase my question in hopes I get this right.

                        a) Unfortunately we don't have the same products available here than you do. Therefore recommending your favourite and trusted brand names won't help me. But thanks for trying!

                        b) What would you use under a waterbased acrylic paint? There are two "cases". I want to lock the raised grain to be able to sand it off. Seems to appear a lot in pine - too soft to be removed by sanding alone. Or if I do have a smooth surface, how can I prevent the grain from raising when painting? Diluted paint-sanding-paint?

                        c) What would be your choise under polyurathane varnish? The same varnish thinned with mineral spirits?

                        d) What could I use under stains? I'll be using birch plywood a lot and after some experiments I'm all ears.
                        *I tried water based stain. I got relatively even colour, but it raised the end grain, also in the xx holes. No way I'd be sanding them smooth!
                        *I tried spirit based stain. Got blotchy and very ugly result. It was both unevenly absorbed and dried too quickly, hence leaving brush marks.
                        *Tried wax based stain. Returned the can to the shop. May have been a faulty item, I don't know. It didn't penetrate the wood at all, it was all black grey pigment dots on the surface. Their sample was even grey brown.
                        *Oil based stain - I haven't found any.
                        I guess the sanding sealer hint depends on the type of stain. Right?

                        e) Finally, my English fails me.
                        Originally posted by GrayBeard Phil
                        One of the most common home brew sanding sealer is a 1-lb cut of de-waxed shellac.
                        Phil, please put this in other words. I recognized the word shellac...



                        • #13

                          I am red-faced I didn't realize that you wouldn't know ---

                          A 1 pound cut of shellac is 1 pound of flakes mixed with 1 gallon of denatured alcohol. A three pound cut is 3 pounds to a gallon alcohol. In round numbers, say 1/2 kilo to a liter of alcohol is a-bit over a 1 pound cut, 2 kilograms to a liter, I guess would be about a 4 or 5 pound cut. Normal finishing top coat would use a 4-lb cut. I hope someone will check my math on this, I could be way off on my English-metric conversions.

                          To many users, shellac comes in flakes and is sold by the Imperial measurement of pounds. See for lots more info.
                          I don't know about your part of the world, But I guess it is sold in Kilogram packages. The flakes are sold as Waxed and De-Waxed (wax removed). Then you select the color from deep amber to Blond. All shellac has an amber-yellow tint to the finish; it is how deep you want the amber-yellow tint.

                          You mix the flakes with natural alcohol (IMHO: not isopropyl-alcohol which comes from petrol chemicals also called crude oil) In north America, there is a tax on alcohol that is sold that can be consumed as a beverage. To avoid the TAX man, the ethyl-alcohol is made poisonous and cannot be changed into a drinkable liquid. This is called Ethanol or Grain Alcohol. The act of making it poisonous and quite toxic makes it Denatured Alcohol (term comes from "against" nature) or sometimes sold in USA as Shellac Thinner.

                          Aside: I grew up being told that the term used in the middle of USA "wood alcohol" was used to describe alcohol made poisonous so 'wood workers' could continue their trade of furniture making in Michigan, Wisconsin and Minnesota. Later I learned that the term came about because they used wood saw dust to brew the alcohol instead of grain like corn or wheat and is correctly called methyl-alcohol. methyl-alcohol will dissolve the shellac flakes but with an evaporation rate (drying time) faster than ethanol. Methyl-alcohol is also toxic.

                          Just in passing, I have found that most fret work I do ends up being touched or held in someone's hand. Shellac is not user friendly-- it is a 'soft' finish that does not take well to being touched a lot.

                          I hope this helps. Do Google on shellac for more information. Because, and trust me on this, the art of dissolving the flakes, straining the shellac, is messy and is really an art to get it just correct. And never mix up shellac with a cat around, that is just asking for extra mess to clean up. And don't even ask about French Polish technique -- you have to see a video to understand that insanity.



                          • #14

                            To answer some of your other questions:

                            Water based lacquer does not need a special sanding sealer as a primer. Use a diluted version of your finish as a sanding sealer. Let the sanding sealer cure (not dry, let it cure) for an extra day or so then sand smooth.

                            Be aware, many have given up on brush applications of finish for scroll saw work. They use spray cans with many light coats, which controls the running inside the fretwork cuts. Or just dip the project into a disposable tray with Boiled Linseed Oil (BLO) first coat a 50% / 50% mixture with solvent (mineral spirits) and full BLO after that. Note: BLO drys quickly, but can take up to several months before it is fully cured.

                            BLO can be used as a sanding sealer, but it will alter the color of a pigmented stain. BLO, after you wipe it off the project, is a thinner coat of finish and may not hold up the fibers as well as you seem to want for sanding.

                            - Acrylic paint needs a primer paint coat. Acrylic paint looks bad because of the pine absorbing the water solvent faster in places than in others. There are acrylic primer paints sold for high cost in art stores. The primer paint creates a water barrier so the acrylic paint can slowly dry and cure. Primer paint is not a sanding sealer, but can be used as one if you apply a second primer coat.

                            -use a 50% / 50% poly-to-thinner ratio for polyurethane varnish provided you cannot get a can of sanding sealer from the same maker as your varnish.

                            -ON some scrap wood, try BLO as a sanding sealer, sand, then try you stain and then apply your top coat. (many time the true color of stain won't be seen correctly until top coat is applied. See how the color looks when finished. You may not like it at all.




                            • #15

                              Its not really complicated, you want to keep it simple as possible.

                              I will go down the list, and see what will work for you.

                              To seal the WB acylic paint, to keep it simple, if you can use the same clear coating that you will use to protect the final finish thats the way to go.

                              As an example: You could use an Acrylic Lacquer as your sealer, and then you can reduce the paint into a stain, or use it as a paint, be sure you allow everything to dry, and then apply the clear acylic lacquer as your topcoats. These would all be compatiable. The sealer and the topcoats must be compatible.

                              Regarding, raising the woods grain, all solvent coatings do not raise the woods grains, it is the water base stains, paints, and WB coatings that do. If you use a solvent coating as your wood sealer then the coating must be compatiable with the water bases stains and paints when they are seal in. You must be extra carerfull and always allow the stains, paint, and coating to drt, before you coat or recoat, as this is the cause of many problems in finishing, rushing to complete the work.

                              By using the same sealer and clear coats it reduces your chances of having problems, because of the compatiabilty being the same.

                              Poly mixed with mineral spirits or white spirits are good choices as they are compatibles anf the right solvent for poly.

                              To reduce blotching on the end grain, try sanding with finer grits, and then seal the bad end grains. You could also try using "gel stains" they do not blotch, be sure you wipe them dry.

                              Whenever, your working on different woods, always start out making up complete samples, as on some open grained woods may need two, three, or four wash coats ( a wash coat, is a thinned out coating) while the closed grained woods will take less, you may want to bring out the open grains when your staining, then don't seal off the woods with too many wash coats, or the stain will not have anywhere to go, and it will look like a paint.

                              I hope this helps to clarify the problem.


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