Replies

1. 
   Westchester | Feb 05, 2010 05:47pm | #1

   Sealers are useful to provide a uniform first coat - thin the poly + apply as your sealer.

   A sanding sealer is easier to sand then the thinned poly - but you should be OK sticking with the poly throughout.

   SA

2. 
   SteveSchoene | Feb 05, 2010 09:17pm | #2

   Sanding sealer has very few uses in non-industrial finishing, and very few indeed under single part consumer grade polyurethane varnish. You might use it ONLY if this sanding sealer was made by the same manufacturer AND specifically recommended, by model number, for the particular polyurethane varnish model number. Even then I would only think about using it if it made a very noticable reduction in blotching compared to the top coat by itself.

   In general polyurethane varnish doesn't adhere very well to other finishes. If the varnish by itself 'blotches" more than you want--and some is both inevitable, and in my mind attractive, telling you that the wood is cherry. I would use a very light cut of blonde dewaxed shellac instead of a sanding sealer.

   I would also consider using a better varnish than a wipe on polyurethane. Almosst all of the poly varnishes are made with linseed oil which yellows relatively much over time.

   To keep the varnish light in tone for less blotching, and to reduce yellowing over time, a varnish made using soya oil would be desirable. I know two such varnishes, Pratt & Lambert 38 and Cabot Varnish (8000 series, not the more readily available Cabot Polyurethane) These non-polyurethane varnishes don't have the adherance problem of the poly, and will be more than durable enough for a wall unit.

3. 
   HowardAcheson | Feb 06, 2010 11:46am | #3

   I agree with Steve about the non-need for "sanding sealer". In general it's an expedient for production finishing. It's intent is to fill minor imperfection in the wood with a high build material. However, the stearates (soaps) are very soft so that they sand easily. But, they also do not allow many finishes to adhere properly. There are some oil based "sanding sealers" available but the lacquer based are to be particularly avoided if oil based poly varnish or waterborne finishes are to be applied. Sanding sealers have little use in reducing the uneven coloring in wood such as cherry.

   Much better is to apply first a coat of your planned final thinned varnish finish. You are much better off to make your own wiping varnish. Mix your varnish 50/50 with mineral spirits. Apply the first coat, let dry and lightly sand with 320 paper on a felt surfaced sanding block, sanding by hand in the direction of the grain. Vacuum off the dust and then a couple of more coats.

   Here is something that should help. A friend of mine put it together years ago and it has worked well for many.

   QUOTE

   There are a number of suggested application regimens that are totally subjective. The number of coats in a given day, the % of cut on various coats, which coat to sand after, when to use the blade and a whole host of other practices are all minor differences between finishers. There are some things that I consider sacred when applying a wipe-on finish.

   First, you can use any full strength oil based clear finish. Polyurethane varnish or non-poly varnish is fine.

   If you are making your own wipe-on the mix is scientific - thin. I suggest 50/50 with mineral spirits because it is easier to type than any other ratio and easy to remember. Some finish formulators have jumped on the bandwagon and you can now get "wipe on" finish pre-mixed. If you use a pre-mixed, thinning is generally not necessary. But making your own is cheaper and you know what's in it.

   The number of coats in a given day is not important. Important is to apply a wet coat with an applicator and merely get it on. Think of a 16 year old kid working as a busboy at Denny's you have sent over to wipe off a table. Sort of rub/swirl the the material on like you would if you were applying a paste wax. Don't attempt any straight strokes. The applicator should be wet but not soaked. The applicator can be a non-embossed paper towel shop towel, half a T-shirt sleeve or that one sock left after a load of washing. Once applied,leave it alone. The surface should not be glossy or wet looking and, if applied correctly, there should be no "brush stroke" type marks. If you have missed a spot, ignore it - you will get it on the next coat. If you try and fix a missed spot you will leave a mark in the finish.

   Timing for a second coat involves the pinkie test. Touch the surface with your pinkie. If nothing comes off you are ready for another coat. If was tacky 5 minutes ago but not now, apply your next coat just as you applied the previous coat. Remember, you are wet wiping, not flooding. After applying the second coat, let it fully dry for 48 hours. Using 320 paper and a sanding block lightly sand the surface flat. Now, begin applying more coats. Do not sand between coats unless you have allowed more than 24 hours to elapse since the prior coat. The number of coats is not critical - there is no critical or right number to apply. For those who need a rule, four more coats on non-critical surfaces or six more coats on surfaces that will get abraded seems to work.

   After your last coat has dried at least over night you will have boogers in the surface. You should not have marks in the surface because you ignored application flaws. You may have dust, lint and, if you live in Texas, bug legs. Use a utility knife blade at this point. Hold it between your thumb and forefinger, near the vertical, and gently scrape the surface. Gentle is the important word - no harder than you would scrape your face. If you start scraping aggressively you will leave small cut marks in the surface. After you have scraped to the baby butt stage gently abrade the surface with 320 dry paper or a gray ScotchBrite. Clean off the surface. Now, leave the area for two hours and change your clothes. Apply your last coat with a bit more care than the previous coats and walk away.

   An anal person is going to have a tough time with this process. Missed spots have to be ignored. Wet wipe, don't flood. Scraping to babies butt smooth means scraping no harder than scraping a babies butt. Ignoring any of these will leave marks that are tough to get out. Getting these marks out requires some aggressive sanding to flatten out the surface and starting over.

   Jim Kull

   END QUOTE

   Finally, It works better to use a gloss varnish for all coats except the last. The flatteners in semi-gloss and satin tend to rapidly fall out of suspension when the finish is highly thinned. If you want a non-gloss finish, use it only on the final coat or two and be sure to stir the material frequently or you will end up with cloudy streaks.