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Thread: sanding between coats of varnish

  1. #1

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    Has any one tried sanding between coats with a stainless steel scourer, for pots and pans, so that it will not leave any wire wool remains to show up as rust later on?
    regards harry.

  2. #2
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    Use sandpaper. The reason for sanding is two-fold:
    1) to help create a mechanical bond between the old coat of varnish and the new.
    2) and to take down the high points of the surface so each coat as it goes on is flatter than the last and you will get a mirror-like finish. Any kind of bronze wool or stainless steel wool will not perform this second function.

  3. #3
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    Your point about a stainless steel scouring pad over steel wool re rust is headed in the right direction, but a stainless steel pad is WAY WAY too rough.

    3M Scotchbrite pad is more what you want if you go this route. Even in these pads there are different "grits" but the red Scotchbrite seems to be the pick.

    Happy Sanding.
    - M

  4. #4
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    I use wet or dry sandpaper (wet)in progressively finer grits varnish, 220, varnish, 220, varnish, 320, varnish, , 400, varnish, 450, varnish, 600, varnish. The water keeps the grit clear and the varnish slurry lubricates as you sand so you get a finer finish.

  5. #5
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    What TimH said. And what Margot said, but Scotch brite won't level - only sandpaper.
    As a former professional finisher/chemist, I've never gone finer than 320 (dry) and always recieved museum quality finishes - never had sand-scratch lines appear. Finer than this, and its possible to impair adhesion of the final coat.

    The real trick to the great topcoat, is to let your full-build varnish dry for a few months (more or less depending on sunlight exposure) so that the varnish can off-gas and terminate the molecular chains, then do one of two things: add an additional coat, or wet sand (now you can use the 800 - 1200 grit paper) level, and buff to mirror flatness and gloss.

    If you immediately go through all the work of superfine sanding between coats, and even rub-out the final, it will all be wasted time when the sunlight causes further evaporation of unreacted monomers and trace solvents, as well as additional cross-linking.

    I always let the finish "stabilize" for a while before doing final buff and polish.

  6. #6
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    John,

    That finish looks great. Is there any reason to think that polishing the cured varnish would make it less durable? Does the polishing affect the physical properties in any way?

  7. #7
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    At the marinas where I used to work, polishing was never a consideration. Sometimes you had to put an extra coat on though to get a perfectly dust-free gloss on the final coat.

  8. #8
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    Originally posted by Scott Rosen:
    "Is there any reason to think that polishing the cured varnish would make it less durable?"

    I think it could, in the presence of UV or by reducing the film thickness to a level of it now being too flimsy. The use of fine gloss additives (like clays amongst others) that come to the surface to help create the gloss, can be removed (not that this a drama), and the UV retardants and stabilizers that do the same, can be reduced, this could be a drama.

    Warren.

    [ 05-26-2004, 09:26 PM: Message edited by: Wild Wassa ]

  9. #9
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    Good question, Scott and Warren. I'd say that it depends on the particular brand/formula. Some may have slip/mar-resistant agents (polysiloxanes) that migrate to the surface during cure stage, that increase crosslinking at the surface, and/or leave a microfilm of "slickness". Some of these agents DO react and bond to the varnish molecules, but many do not, and can be worn off over time.

    I have a hunch that most marine varnishes don't have these agents (expensive, 2K urethanes might), so removing a few microns of finish during rub-out won't make much of a difference.

    Rubbing out a finish certainly isn't going to add durability, but always remember that you are creating a new skin when removing the top few mils or microns during rub-out, and this new surface may be soft initially (cuz it was just buried in a "moist" matrix of sub-surface polymer), but will also harden over time, like the topcoat did, and likely arrive to at least 90% of the former durability.

    I always like to add one more final spray pass, knowing I'll remove some during buffing. I've rarely ever layed down a coating that was dust-free, so I generally rub-out only the "glorious" areas of a finish, like the foredecks, and simply locally buff any bad spots elsewhere.

    I'm also assuming that one will only remove up to one to two mils from a 10 - 15 mil+ finish. Any more than that and you do reduce the UV protection. I buff only to remove orange peel, sags and dust nerds which is about 1/2 a mil average (12 microns). Sags and runs can be 30 mils thick, so I razor them off only partway, so that the finish can dry before I wetsand/buff them.

  10. #10
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    Any advice on brands of varnish, i.e., is "spar" varnish from the hardware store (Ace, Man-O-War, Minwax, etc.) adequate for most trailer-sailer small boat projects (trim varnish)? What is the advantage of more expensive "marine" brands (epifanes, interlux, Captain's Z-spar, etc)?

  11. #11
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    I've never gone finer than 320 (dry) and always recieved museum quality finishes - never had sand-scratch lines appear. Finer than this, and its possible to impair adhesion of the final coat.
    I'm with John, except for the museum quality part. (results will vary [img]smile.gif[/img] ) I've only done one real buffing compound job, on a Piano (not a Steinway [img]smile.gif[/img] ). I do, however, lightly rub a freshly air-dried, near-final surface with a soft wool rag. It seems to pick up any particles that find their way to the wet varnish.

    Anyone got any comments on wetting out the "final" coat ? The one applied after the long curing interval.

  12. #12
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    I don't know if this trick is suitable for boats or not, but my Dad heats up a pot of water on the stove and then puts a can with varnish in it, in the hot water, not on the stove anymore

    It helps the varnish run better, to remove brush marks. He did it when varnishing a floor with Estapol. Might be useful in some situations, not sure about the more modern varnishes used on boats though [img]smile.gif[/img]

    I'd like to be able to offer better advice, like if I actually had the time to build a boat, ya know

  13. #13
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    Originally posted by Vincent S:
    "What is the advantage of more expensive "marine" brands ... "

    More plastic, more UV stabilizers (and hopefully more pigment if coloured). Quality pigments are good UV retardants.

    Originally posted by Paul S:
    "Anyone got any comments on wetting out the "final" coat. The one applied after the long curing interval."

    Give the old surface a uniform sand, I like 400 before topcoats. I would be inclined to put a very small amount of a quality oil, like refined linseed oil in the top coat, or reduce the solvent level (if that had been altered), so that the new paint has slightly more oil than the previous coat. The underlying coat will draw oil from the top coat slightly. This will increase the longevity of the new paint and keep it flexible for longer, the top coat will not loose needed oil, and this also rejuvenates the old coat. You will read, "the more oil the better" on manufacturer’s sites. Too much oil slows the drying, greatly.

    I haven’t tried Penetrol added to varnish. Adding Penetrol will do the same as the oil, the flow control and levelling are enhanced using Penetrol, with no real change in drying time, as the bonus. I must try Penetrol in varnish. A flow control medium like Penetrol is a very good additive I find for oil based materials. Floetrol for water based.

    Varnish is an fine material, ... to strip. Long live real varnish. Poly users beware.

    Warren.

    [ 05-26-2004, 11:22 PM: Message edited by: Wild Wassa ]

  14. #14

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    Thanks for all your input lads, by the time this boat is finished I had better be as expert as you are on this subject. I will try all your tips and will try and get some photos done for you all to see. Have to find out how to get them on to this site and send them!
    regards, divine wind (kamikasi)

  15. #15
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    I have very good results with Penetrol in varnish. Used it for some time. I add about 2 oz to a full quart.


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