How Do I Apply PU Varnish

Discussion in 'Wooden Boat Building and Restoration' started by rxcomposite, Apr 9, 2011.

  1. rxcomposite
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    rxcomposite Senior Member

    I tried to widen my skill by learning how to finish. The project, a plywood tabletop was my test piece.

    I sanded it smooth, oil stained it, dried it for a day, then applied Polyurethane varnish. The varnish flowed well but after it has dried, it looked liked this. (I sanded it to make it more obvious) It seems the first coat pulled together to form puddles or valleys and would require a lot of sanding.

    What did I do wrong? The pros can do it it in two coats and get a nice perfect finish but mine looks like it will need 3 or 4 coats.

    Do I apply it thick during the first coat, then thin down the suceeding coat or vice versa.

    Some said I should have used a sanding sealer but the instruction in the can did not specify.
     

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  2. michael pierzga
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    michael pierzga Senior Member

    A typical yacht varnish job, from bare wood with Epifanes products, would be to build up coats to seal the grain...green on green..with Epifanes PP Sealer. PP is a runny, thin fast dry varnish

    PP can be overcoated after three hours without sanding....so three coats of PP today. In the morning the surface will not be flat so sand the project flat with fine paper...perhaps 220 and once again...three coats PP, green on green. PP can be applied with a foam roller and brush. Now the wood grain should be filled with the 6 coats you have applied.

    Next take a top coat quality... UV protected... varnish. Epifanes makes a good Pu varnish. If its a flat surface roll and brush with the varnish thined say 15 percent. You should get a very good surface...let the coat cure overnight then sand imperfections and brush strokes flat with very fine paper 320, then sterilize the surface , yourself , your paint tools and work area. Lock down the paint room then spread a 20 percent thinned topcoat. Should come out like glass. To minimize dust particles...if possible orient your horizontal surfaces vertical.

    Epiphanes PP sealer ...tec sheet http://www.marineware.com/pdf/Epifa...ac9e64972724=ccc32179a8b8f0b7b913c3ebf9ee221d

    Awlgrip also makes a nice PU varnish. If you know awlgrip products you will appreciate the brushabilty and wet edge working time of the catylast and thinner system.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Okay, lets call an apple and apple folks. Polyurethanes are not varnishes, even though the term varnish is used for about anything clear now a days. Polyurethanes aren't applied like varnish, don't generally look like varnish, don't act like varnish and polyurethanes are divided into two general categories, modified alkyd and LPU. These are further divided, but doesn't affect the application techniques.

    The only time you want to thin a clear coating of any type is when it's going on raw wood, period. On no other occasion does thinning any type of clear coating make sense, unless you are reducing to spray, which is a different animal all together.

    RX, your issue is normal and you shouldn't be discouraged. You have brush marks and it's no big deal. Unlike varnishes that dry slowly, so that the material applied to a surface can "self level", polyurethanes dry quickly enough that it might not self level, before it begins to gel up (harden). Most polyurethanes have "wetting" agents formulated into them to help the "flow" of the material after it hits a surface. These permit the polyurethane to self level. You can improve the flow of both types of polyurethanes with Penatrol (alkyds) or a wetting agent (LPU's). Of course this comes with some cautions, mostly, don't use to much. as they can dull and/or cloud the finish.

    In your case, apply a thin coat of polyurethane after scotch brighting the current surface (in the direction of the grain). Use a roll and tip method or just a brush as preferred. Temperature is important and it looks like you may have been in cool temperatures when applying the finish. This will harm the self leveling properties of polyurethane.

    Once the next coating is dry, block is down (sand with the grain) with a long board, which will remove the freshly applied coating from the high spots, but will leave it in the lows, building up bulk there. You may need more then one coat to completely fill the lows.

    Once the surface is level again, you are starting over so to speak. Now, apply the finish THIN and uniform. The easiest way to do this is by rolling and tipping the surface. Rolling applies material uniformly, but don't try to "stretch" the material by over rolling or working too large an area per roller full. Also don't over brush the material, just knock down the stripling left by the roller. This is a light touch with a fairly dry brush. Also foam brushes work a lot better then natural.

    Applying clear finishes is all about technique, applying thin, uniform thickness coats and of course working clean in the appropriate temperatures necessary.

    The LPU's (two part polyurethanes) are more difficult to apply and usually require more additives to get the viscosity right. This finishes are very hard and durable, plus have excellent gloss retention. Single part polyurethanes are easier to apply and have nearly the same level of preformance.
     
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  4. rxcomposite
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    rxcomposite Senior Member

    Thanks Michael. PAR.

    I am using a 2 part Hudson Polyurethane Varnish as it says on the label. These are the leftovers from the guys who did my wooden floor. It is a water clear topcoat. Like PAR said I think we should stay away from using “varnish” because the only varnish I can remember is Valspar.

    I applied the 2nd coating unthinned. That filled up the highs and lows as can be seen from the picture. Still needs a little sanding.

    On the second panel, I thinned it. It is showing brush marks but it isn’t much of a problem. Still needs sanding.

    On both instances, I just have to work quickly because the coating gels within 2 to 3 minutes and I can’t go back and brush over it to correct my mistake.

    Well, It seems it is really a 3 or more coating process. Lots of elbow grease “sanding” in between coats. I have seen craftsman on the yachts we built giving it a 6 to 10 coats of very thin layers. Beautiful finish though.

    What I am not sure of is the sanding sealer really necessary or do I have material incompatibility? The first coat was as I have said, laid on evenly but as it gelled, the surface tension broke causing the highs and valleys. Was the 1st coating not sticking well to the oil based stained surface?

    Thanks for the tip on the foam brush and scotchbrite PAR. I have seen craftsman using a ball of cotton wrapped in lint free coat for the final coating and very fine grit sandpapers in between. Seems an old trick by the oldies.

    Still a long way more before I graduate to more expensive wood and finishing.
     

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  5. michael pierzga
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    michael pierzga Senior Member

    Looks good to me. First couple coats are always crude.

    Work Green on Green...build up a good base as rapidly as possible...sand flat , then topcoat.

    To apply a topcoat your brush must glide...not pull. Add as much thinner as is necessary to get your paint brush to glide. Thinner also extends the wet edge working time and allows the varnish to flow out smooth as is cures.

    As for compatibility normally you see fish eyes form.

    The test for compatibility is the peel test with a piece of fabric tape.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Rx, the process of applying clear coats is two stages: bulking up and finishing. The bulking stage is as it sounds, just building up film thickness so you can get to a point where all the sanding is only affecting the film, not getting down into the substrate. This process lets you level out the surface as you go.

    On the initial coats I don't even bother with major smoothing attempts, just get some film thickness down. After the 3rd or 4th coat, I'll then level the surface, knowing full well that I'll sand through 2 of the coats in the process. Then comes the finish process which is where control of the coating is the only thing that will save your butt. The finish coats have to go down thin and smooth. Novices tend to over brush, don't keep a wet edge and try to work too large an area at once. These mistakes will produce brush marks, drag marks and start and stop marks.

    You'll get it, but the learning curve is full of runs, drips, brush marks and sags.
     
  7. rxcomposite
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    rxcomposite Senior Member

    Thanks again Michael, PAR.

    I have the 3rd coat on and have sanded the surface. I have to wait for a few days because the surface was a little gummy to sand. Now it is powdery when I sand it.

    I think I have got it now. Build up the bulk, sand level and apply the thin layer topcoat.

    I bought a better brush to make it glide easy. My old one is a little stiff and I was applying it like paint. This "varnishing" is a lot harder than painting but I am getting there.

    Again, many thanks. If not for the advice, I will forever be experimenting.
     
  8. michael pierzga
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    michael pierzga Senior Member

    I always use cheap throw away chip brushes and foam rollers. Even the finish coat. Pick up a handfull of the widest throw away chip brushes in the shop.

    If youre only tipping after the roller they work great. Give them the vacum cleaner treatment ,to pull out the loose hairs and junk , before use.

    No Need spending all that money, time and solvent to clean a quality brush.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I agree in the throw away brushes thing, but chip brushes are notorious for dropping hairs, even if you vacuum them. I've tried everything to prevent this, including using a wire brush on them to pull the last few remaining loose hairs, but inevitably, they fall out and you don't see them until it's cured, ruining an other wise perfect finish. Foam brushes don't have this problem and they produce a much smoother stoke. They don't hold much paint or varnish, which is just what you want in a roll and tip job. Buy the foam brushes by the case and save big bucks. You'll use them like prophylactics in collage.
     
  10. Petros
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    Petros Senior Member

    All good advice. I notice many pro boat finishers using foam brushes now. Also, some books claim seven coats minimum for proper protection of wood surfaces. So you have a number more coats to go yet.
     
  11. michael pierzga
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    michael pierzga Senior Member

    Foam brushes are very worthwhile to have as part of your paint kit but are unfortunately unavailable in many parts of the world. Locally they cost 3 and a half dollars apiece. Foam brushes are great for flat surfaces when spreading thinned paint but are handicapped when " cutting in " , dealing with inside corners or overcoating a panel that is loosing its wet edge.. The real beauty of foam brushes is cleanliness...no dust or broken hair shedding ...so a superior finish coat..
     
  12. rxcomposite
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    rxcomposite Senior Member

    Foam brushes are the new invention to replace the cotton balls on post no. 4. The cotton balls are throwaways. The old pros make up/use about 6 to 8 in a day. But I agree, it is hard to find. I still cannot get some. Lots of foam rollers though.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Mail order, by the case . . . just have it drop shipped directly from the supplier . . . UPS or Fed X: amazing . . .
     
  14. cthippo
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    cthippo Senior Member

    I use a high quality polyester brush for applying PU and clean it in one of the new "green" low VOC thinners (which also happens to be cheaper than conventional) and it has lasted me well over a year. The chip bristle brushes drop bristles all over the place and so i won't use them unless I can tolerate that.
     

  15. rxcomposite
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    rxcomposite Senior Member

    The PU topcoat I am using is a shiny finish. I also like the satin finish and am trying to see if I can do it.

    The first picture is the original satin finish from the guys who worked on my furnitures. It seems it was sprayed because there is no brush marks and the finish is light and even.

    I used the leftover, a single part, dark colored topcoat. It was already thick so I used lacquer thinner to thin it down. I brushed it on (2 coats) and it seems I do have the satin finish but not so at closer inspection. At extreme closeup, you can see what appears to me as micro bubbles.
     

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