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Sanding Between Coats?

Discussion in 'Tips and Tricks' started by crosscuts, Feb 23, 2010.

  1. crosscuts

    crosscuts LOVES Wooden Canoes

    Sanding between coats of varnish or paint helps level the surface and removes settled dust and bugs. IS THERE ANY SCIENTIFIC OR MECHANICAL EVIDENCE THAT SANDING IMPROVES THE ADHERENCE BETWEEN COATS OF THE FINISH ITSELF?

    Finishing raw wood I usually put on two and sometimes three coats before doing any sanding. The reason for this is that it saves sanding time, and adds buildup for more effective sanding. I have never experienced a failure of adherence between multiple coats of finish even after years of use in varying conditions of exposure to moisture and sun.

    R.C.
     
  2. Andre Cloutier

    Andre Cloutier Firestarter. Wicked Firestarter.

    Dont know about the science of it but I hot coat 2 or 3 as well before any sanding , havent had any problems to date. last project has 3 years of use with no problems. Dont think varnish is as fussy as paint when it comes to adhesion, at least Captains hasnt proven to be.
    I usually rely on Todd's replies for the science of why or why not to do it so I may change my mind yet..
     
  3. Michael Grace

    Michael Grace Lifetime Member

    I'm no coatings chemist, but varnish is an organic material and I practice biochemistry every day. I also run a high-resolution microscopy laboratory, so I spend a great deal of time on electron microscopes. What I see there likely applies here.

    The prevailing wisdom is that sanding creates scratches, and therefore creates much greater surface area and a more complex microarchitecture of the surface to which the next coat of finish will be applied. Even so, the dimensions of this microarchitecture are much greater than the molecular dimensions of the finish. Therefore, it seems unlikely that scratches alone are responsible for adhesion. In other words, adhesion of one layer to another is not just mechanical. The solvents in a finish that is compatible with a previous finish (such as one coat of varnish over another) can dissolve a bit of the surface of the previous coat, thereby allowing the two layers to intermingle and cure together. "Hot coating" works because the previous layer of finish is incompletely cured. Because total cure time is long- much longer than the time-to-next-coat listed on the can- hot coating should be doable for a long time, with increasing time after the last coat meaning less and less ability of the new coat to merge with the surface of the old one. Thus, I believe that sanding scratches, by creating this complex microarchitecture, do two things: (1) scratches increase the amount of surface available for a bond between the two coats, and (2) they open up the previous layers of finish that may be less fully cured than the surface (at least within the first year or so). Increasing bond area by increasing the complexity of the surface should become increasingly important over time after the previous coat is applied.

    This is not to say that mechanical adhesion is unimportant. Finishes that polymerize as opposed to simply drying will eventually fully polymerize and subsequent layers can no longer solubilize even a small amount of surface. In this case, the microarhitecture of sanding scratches should prmote mechanical adhesion.

    There's one more thing that applies particularly to older finishes. Surfaces, especially as they get older, become increasingly contaminated, and some of that contamination is not just sitting on the surface, but rather chemically bonded to the surface. Sanding cleans that old surface (well, removes it), removing impurities that may be well bonded with the old surface.

    Bottom line? Yes, hot coating works (I usually do exactly what Andre describes), and "less-hot" coating- longer time between coats- should work as well, but it should become less effective as a finish gets progressively older. But as this happens, sanding still produces a more complex surface for mechanical adhesion of one coat to another. So as a finish gets older, sanding becomes increasingly important for promoting chemical bonding if still possible, and mechanical bonding in any case.

    Michael
     
    Last edited: Feb 23, 2010
  4. Todd Bradshaw

    Todd Bradshaw Sailmaker

    What he said....
    I sometimes hot-coat varnish, but generally don't on paint. If I get a paint job that looks good, I'll usually let it dry and then sand or Scotchbrite it the next day for fear of screwing it up, messing with it before it's had some time to harden a little bit. Whether I hot-coat it or wait and sand can also depend on how late in the day it's getting to be and what the temperature is doing.

    I have noticed though, that some finishes tend to go on smoother over a sanded surface than they do when hot-coating. I had one boat that I sprayed. The varnish can said that within a certain time frame, no sanding was needed. I tried it and got an awful lot of orange-peel. I did it again, hoping to get a smooth final coat and the same orange-peel thing happened. It wasn't until I finally sanded it, knocking the gloss off, that I could spray another coat and have it level out properly. Don't know why.
     
  5. Tom Widney

    Tom Widney LOVES Wooden Canoes

    Sorry guys but after reading all of the preceeding coments, I'm not sure that I understand exactley what you all mean by the term "hot coating" is it simply applying another fresh coat of varnish with out letting the first coat cure properly or simply not sanding between fresh coats?
    Tom
     
  6. Michael Grace

    Michael Grace Lifetime Member

    Sorry about that, Tom. Yes, you are correct. "Hot coating" refers to the application of another coat of finish before the previous coat has thoroughly set up, generally without sanding the previous coat. The idea is that the subsequent coat will readily meld with the previous one without the aid of having the previous coat roughed-up by sandpaper. Value? Speed of getting on multiple coats without the necessity and tedium of sanding. Problem? Aside from the issues of leveling and removing trapped dust and insect parts, building up coats too quickly can lead to failure of the finish. Too little cure time between coats causes trapping of solvents in lower layers of a built-up finish, which in turn leads to a skinned-over surface, long overall cure time, and collapse and wrinkling of that skinned-over surface as the lower layers cure (just as a large run or puddle behaves in a finish). This all means that sanding has an added value- if you're unsure whether the previous coat is sufficiently cured to allow subsequent coats, sanding will tell. If it produces a nice white powder (I believe it's called "swarf"), the previous coats is ready for the next one; if the paper gums up, more cure time is needed.

    Sorry if this is more than you needed, but yes, hot coating is the rapid build-up of coats without sanding between (perhaps so rapid that your sandpaper might gum up if used, but hopefully not so fast that the finish suffers from trapped solvents).

    M
     
    Last edited: Feb 24, 2010
  7. Tom Widney

    Tom Widney LOVES Wooden Canoes

    Thanks Michael,

    Don't apologize. This forum is like taking a graduate course in wood working and for a very nominal cost of the annual dues. These days with corperate greed being all thats on the news I'd say that this forum is a lot of bang for the buck
    Tom
     
  8. MGC

    MGC Scrapmaker

    To sand or not to sand?

    Ay, there's the rub...

    Since this is largely speculation, here is another point to consider. What happens to the sanding dust? Presumably you remove this with a tack cloth before you apply the next layer to remove any small particulate from the "micro-architecture".
    I expect that unless a really course grade of paper is used that it is likely that the micro-particles from sanding fill the micro-architecture negating any value from the sanding. Since these sanding particles are not adhered, they may actually promote a looser bond.
    I always clean with acetone between coats to lift the sanding powder (I do sand) from the sanded canvas. I am not sure if that's the correct thing to do but it's my method.
     
  9. Dan Lindberg

    Dan Lindberg Ex Wood Hoarder

    I would 2ed Todd's comment's/observation about paint/varnish going on smoother over a sanded surface vs an unsanded surface.

    I've noticed that if I don't sand, with either paint or varnish, the new layer tends to "bubble up" or "seperate" more, I have no idea why.

    Dan
     
  10. Todd Bradshaw

    Todd Bradshaw Sailmaker

    Acetone has a fairly nasty destructive effect on some coatings, so I would tend to be pretty careful with it unless you're absolutely sure that it won't attack and soften the previous coat. In many cases, it's great for removing some of the common finishes, but what it leaves behind for a surface is no longer what you would consider structurally sound enough to paint over. If you want to use a solvent for cleaning, naptha might be a safer bet. That's what many of the guitar finishers/refinishers use for cleaning prior to applying new finishing stuff. Personally, I've seen too many cases of contamination from solvents and chemicals to ever feel very comfortable using them between coats of paint, resin or varnish. Sanded, vacuumed and sometimes tack-rag-ed seems to work fine. If it leaves anything in the little micro-grooves, at least it's the same stuff I'm applying, so I don't have to worry about compatibility or adhesion problems. Some of the new water-based finishes even caution against using a tack-rag because of contamination problems from the mysterious sticky stuff that they're soaked in. At $40-$50 per quart for some of these instrument varnishes, I make sure to stifle my creativity and follow the directions very carefully.
     
  11. Dave Wermuth

    Dave Wermuth Who hid my paddle?

    another option

    I just finished using a liquid sanding product from Easy Strip. They say to apply it with a rag. I used a 3m pad. Then I wiped it off with a dry towel. They say for best adhesion apply paint after it is dry and up to an hour after use. I did. I liked the way the paint went on. Any one else ever use sanding liquid? No dust, I liked that.
     
  12. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I've heard of it being used on old, completely dried paint, but not between coats on fresh paint (which often can't be considered really dry and stable for a week or more). I haven't tried the stuff myself, but there have been a couple times that I was tempted to. Fortunately, I have a pretty high tolerance for sanding. In college, I worked for a sculptor, hand wet-sanding and polishing big, clear, solid resin castings that sometimes contained 10-20 gallons of resin and had to have a perfect, glass-like surface. Sanding a boat is pretty much just a minor annoyance by comparison.

    Did the directions on the can say anything about using liquid sander to prep fresh paint, or just old paint?
     

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  13. Rob Stevens

    Rob Stevens Wooden Canoes are in the Blood

    Hey Todd,
    I followed the link in to "The Ship". It was really a link to a flashback!
    I enjoyed listening to a selection of samples of your music from the early 70's.
    Seems like a lifetime ago, eh? It's nice that the technology is available to re-engineer and re-release old stuff.

    Rob
     
  14. Dave Wermuth

    Dave Wermuth Who hid my paddle?

    right

    it says it is not for use on fresh finish. This was the top and final coat and the other paint layers had seasoned for about a month. I did not use it in between other coats. AND I used it on a 'glass tub that I found by the side of the road and am fixing it up and giving to my sister. So, there was not much risked. Except after reading the warnings of cancer risk. I think that a wet sand with 220 would be better. The stuff says to paint within an hour so there must be some chemical prep going on too.
     
  15. Todd Bradshaw

    Todd Bradshaw Sailmaker

    That's what I suspected. Fresh paint is probably usually soft enough and still contains enough of its own solvents that the liquid sander might do a bit too much of whatever it does.

    These days, I do most of my between-coat "sanding" with those green (finer grit) Scotchbrite pads. They don't fill as fast as paper and you can wash them out and get more life out of them. If the surface is lumpy, they tend to polish the lumps and you're better off using paper and a block to cut them down and level the surface, but on something reasonably smooth they seem to work well, either wet or dry. If you ever want to really polish something, the packs of Micro-Mesh are worth buying and the assortment will go from 1500-12,000 grit in small steps. I buy the 5" disks and use them by hand (usually wet and with a block on flat surfaces). It takes a while with all those grits to go through, but once you're done, all that's needed is a rag with a little bit of polishing compound and you have a mirror finish.

    http://www.woodcraft.com/Catalog/ProductPage.aspx?prodid=10574

    Rob, glad you enjoyed the music. We're working on a fifth CD, this one with all new stuff, which hopefully will be out by late summer or so. The music business is a lot more fun when you aren't relying on it to put food on the table. Instead, one can have a stable occupation - like making expensive fancy little sails for obscure old canoes in the middle of a recession......
     
  16. Dave Wermuth

    Dave Wermuth Who hid my paddle?

    Todd

    I use the micro mesh from woodcraft for pen turning. There is a Woodcraft store less than an hour from me, down on Van Dyke. The liquid sander takes the gloss off and that's about it. It also does something chemically, I think. that's why they say to paint within an hour of using it. I like the 3m pads. Wet sanding is too messing. Dry sanding is too dusty. I guess that's why painting is one of my weaknesses.
     
  17. OP
    OP
    crosscuts

    crosscuts LOVES Wooden Canoes

    Preparation for finishing

    Airborne dust is a major concern. My shop serves as a bench room, machine room, sanding room and paint room. After sanding I vacuum the surface, (the shop vac exhaust throws up a huge amount of dust), take the canoe outside and blow it off with compressed air, wipe it down with mineral spirits, take a shower, brush teeth, put on clean clothes, wet the floor, tac it, then paint. Back out slowly, close and lock the door and wait until the next day to look at the results. Sometimes it looks good sometimes it doesn't. Repeat the process if necessary. Don't set deadlines, keep at it until it looks right.

    R.C.
     
  18. Dave Wermuth

    Dave Wermuth Who hid my paddle?

    R.c.

    Thanks. I sorta know what to do, but the doing it--I think I get lazy at that point. Dave McDaniel told me how he does his. Similar. My barn has a high roof and lots of dust. I need to just buck up and use my dust collection everytime and on every machine. And then follow your steps.

    When randy and I varnished his canoe last week we were able to get things relatively clean and the sealer and first coat of varnish inside looks real nice. So it inspires.
     
  19. OP
    OP
    crosscuts

    crosscuts LOVES Wooden Canoes

    Dust

    Dave

    If you have the space in your barn create a paint booth with visqueen. Clean it out before painting and wash down the floor. Don't stay inside without some sort of fresh air source, the solvents are nasty. Wear a chemical filter mask in any case.

    R.C.
     
  20. Dave Wermuth

    Dave Wermuth Who hid my paddle?

    R.c.

    I've thought about it. I had a 24' x 32' barn and then added another 24' onto the back.. the perfect area for a paint both. But I have two trractors. 1100 bd ft of cedar and half doz canoes and etc. But With plastic and air cleaner/filter etc I could.
     

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