paint on varnished wooden parts

Discussion in 'Wooden Boat Building and Restoration' started by urisvan, Sep 9, 2012.

  1. urisvan
    Joined: Nov 2005
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    urisvan Senior Member

    hello,

    i am bored of varnishing toereails and sides of coachroof of my wooden boat. can i paint the surface without sanding the old varnish? I know that the ideal solution is to sand the old varnish compeletely and saturate the wood with some layers of epoxy and then apply a polyurehane paint on it. but it is a lot of work.
    Instead of that can i simply paint over the old varnished surface to protect the varnish from the effect of the sun? will it be a proper solution?

    i was using jotun benar as varnish.

    regards
    Ulas
     
  2. Crag Cay
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    Crag Cay Senior Member

    Yes you can paint over varnish, but make sure the varnish is in sound condition and totally covers all the wood before doing so. Patch any bare bits and then lightly rub down and give it all a fresh coat of varnish.

    When that's done, knock the shine off with a light rub with fine wet and dry paper, and overcoat with paint. But the paint mustn't be more sophisticated (rigid) than the varnish. That is, if you have varnished with traditional one pot varnish then overcoat with a single pot paint. If your varnish is a two pack polyurethane over epoxy etc, then you can overcoat it with a two pack paint paint, and so on.

    By getting the varnish 'base coats' in perfect nick before you start, you will ensure that the boat can be returned to varnish at some later date, as the paint won't have touched the wood.
     
  3. urisvan
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    urisvan Senior Member

    thank you for Crag,

    And there are some dark parts on the wood. They don't look like rotten because they are not soft but the colour is black. What should i do for them? Should i sand them until i find the original colour and apply varnish several coats before i apply the paint?

    regards
     
  4. sdowney717
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    sdowney717 Senior Member

    Oxalic acid cleans the wood and restores the color.
    You can buy it as crystals or already mixed into water.
    Wood deck wash which has oxalic acid and some soap mixed in is worth trying.
    Works well on many types of woods.
    So remove the finish in those blackened areas and paint on the oxalic acid.
    It mmay take repeated applications to completely remove the staining.
    It also removes rust stains.

    I find some stains to not be repairable due to fungus having gotten into the wood cells.
     
  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    We used to do that on charter boats when new. The wood gets varnished and then painted on top. When they want to sell the boat, the paint gets sanded off and the wood has a varnished finish.
     
  6. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    gonzo makes a good point. It can be a good idea to always use varnish over the wood and under the paint. That makes it easy to return to a varnished or wood state without those pesty bits of paint pigment that always seems to penetrate extra deep into the wood grain, requiring lots and lots of additional sanding to get out. Consider it sort of a wood sealer and primer. And since you already have that in place, you are already 2 step ahead!

    paul
     
  7. urisvan
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    urisvan Senior Member

    thank you,

    and sdowney717 told about another point. he advised to use oxalic acid to clean the blackened areas of the wood. As i understand from his statement i will sand the varnish over the blackened areas and them with brush put some oxalic acid solution on it, then after waiting for a while rinse it then apply varnish again.

    And i think the most risky part is there must not be any rotten part remain. Because after i paint over the surface, i will not be able to see the wood again so any fung or rotten part can develop under. is it wright?
     
  8. PAR
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    PAR Yacht Designer/Builder

    Oxalic acid is effective on raw wood, not so much on previously coated surfaces, even if sanded. Also this stuff isn't easy to use, if you want a uniform color. Basically it's bleach and applying this in a localized area will just leave you with a different color stain.

    Poke the dark area and check it's density, compared to surrounding wood. If it's softer, then you have rot. If not, it's just a water stain and you can paint right over it.
     
  9. pauloman
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    pauloman Epoxy Vendor

    I hear that epoxy is toxic to mold fungus etc. I'm in the epoxy business, but admit I don't know how true this is. May be PAR or someone else could confirm. If so, a thin coat of thinned epoxy (thin so easy to sand off) has the same pigment barrier as the varnish but with even better bonding Fresh epoxy is often used as a primer. It takes about 7 days to fully cure so if you topcoat in a day or two or three you get a bit of chemical bonding between the not fully cured epoxy and the new topcoat. PLus the epoxy bonds to the wood better than the varnish. In some rare weather conditions - alkyd enamels have drying issues over epoxy with nonyl phenol (common in epoxies).

    personally, I would probably sand, treat black areas with bleach and a tooth brush, let dry, apply varnish or thinned epoxy then the topcoat paint.

    paul
     
  10. PAR
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    PAR Yacht Designer/Builder

    I see no reason to treat areas that haven't any rot in them, if you're top coating with paint. I've read that epoxy doesn't kill mold, etc., but does deprive it of one or more of it's needs, for sustained growth or life itself.

    I tend to agree in that epoxy is a good under coat, as it bonds better to raw wood. This is one of the few ways you can use epoxy as a coating without encapsulation. Under clear finishes, I think it adds to the maintenance issue, but under paint it can help.

    If using an alkyd, use a quality primer to avoid the issues Paul mentions. In most cases paints dry just fine, even alkyds over epoxy, but if your luck is like mine, the one time you elect to skip this step, will be the time it bites your butt.
     
  11. pauloman
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    pauloman Epoxy Vendor

    I think the issue with alkyd enamels and epoxy is based on temp and humidity and nonyl phenol in the epoxy (my best guess after lots of tests) - use an epoxy without nonyl... or like par said, a different primer. Spar Varnish goes over fresh epoxy really really well and doesn't have the sometime enamel/epoxy issue. Of course a pigmented primer shows areas that need more attention and provide a uniform base color - so that's how I lean. I use 'aluthane' (an aluminum filled moisture cured urethane, but I have insider bias, as I stock that product). Even 'spray can' primer seems to work well.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Agreed the conditions and formulations can be a problem and somewhat a crap shoot, as most don't know what to look for. The cheap insurance route is an epoxy primer, which solves the problem. I use several different primers, but I also use primer to finalize the finish, so I'm putting on several coats and knocking them down, until I'm satisfied.
     
  13. urisvan
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    urisvan Senior Member

    hello,

    I scrapped the areas where it is black and the old varnish is coming out easily. But in most of the place the varnish is bonding well so i did not want to scrap it out. Now i have patches of bare wood. So it is not easy to put primer as an undercoat.
    Can i just varnish all the surface without using any primer? Or can i put primer on the patches of bare wood, by that way maybe some primer can come on the varnished wood because the transition of bare wood to varnished wood is not that regular.
    I don't want to scrap all the old varnish...
    I hope i could explain my trouble.

    Regards
     

  14. Crag Cay
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    Crag Cay Senior Member

    Once you are happy the bare patches are dry and the colour you can live with, then varnish them with the same varnish you're going to use, but thin it slightly with whatever the manufacturer recommends. That should soak in slightly and leave a mat finish. Rub any shiny bits with fine sand paper and cover again with un-thinned varnish, allow to dry and repeat until the bare patches look as well covered as the old varnish. Then rub where down lightly and give it all a coat or two.

    Allow to dry, rub down and paint.

    Your are just making good the varnish so all the wood is covered so there's no danger of the paint pigments coming into direct contact with the bare wood.
     
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