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  • Sealing the back of Hardbord with GACApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2017-11-15 19:19:37 ... Most recent comment 2017-11-16 12:26:50
    Acrylic Rigid Supports Oil Paint

    ​This question is directed mainly at Sarah Sands (though anyone can feel free to weigh in): 

    Are there any particular concerns with using GAC 200 to seal the back and sides of unbraced Hardbord panels? I've tested a lot of different finishes for this purpose, and consistently find that GAC 200 performs better than just about anything else in this role. My criteria are two-fold: I'm looking for a coating that will provide some moisture resistance, and also one that will act as a sort of consolidant to strengthen the Hardbord (particularly the edges, which when damaged are prone to fraying). 

    Strengthening the Hardbord is probably the most important concern for me; in general, I don't find Hardbord to be all that reactive to humidity changes, but it can be vulnerable to physical damage, especially when dropped. With a couple of coats of GAC 200 on the back and sides, it seems much more durable. The cured GAC makes the edges very hard and, well, solid. I can drop a panel from eye level onto a hard floor--on a corner!--and it suffers no visible damage. An unsealed panel dropped in the same manner invariably ends up with a dented corner that imediately starts to fray. 

    The fact that the GAC dries relatively quickly is also a plus. I've gotten similar results with oil-based polyurethanes, but they require at least four coats to build to a film on Hardbord (which is very absorbent), and so drying time becomes an issue. Being able to seal the back of a panel in a day (rather than several) is helpful. I've also tried GAC 100, but it dries to a somewhat tacky surface, and does not harden the edges of the panel to the same degree that GAC 200 does. I know that GAC 200 is supposed to be more brittle, but it held up well to my impact tests. 

    So I guess the question is whether or not this is an acceptable use for GAC 200. Also, will it remain stable as a surface coating on the back and sides of a Hardbord panel, or should I top-coat it with something else?

Answers and Comments

  • EditDeleteModerator Answer

    ​Hi -

    Using GAC 200 in this way is fine, just keep in a few things in mind:- it is only water resistant and not waterproof, and certaainly a polyurethane would produce a harder and tighter film. 

    - when using GAC 200 alone and creating a continuous film, we recommend having it dry at a min of 70F. Otherwise there is a chance of  microscopic microfissures forming that are not really visible to the naked eye. We noticed this when testing GAC 200 as a size for blocking oil. When dried at 70F or above the films were succefful at blocking oil, while below it grew increasingly able to allow small amounts of oil to penetrate through.

    - if the above is concerning, you can add some GAC 500 to give some additional flexibility and a lower film forming temperature. A 3:1 GAC 200:GAC 500 ratio would be good. You should still have a nice low tack film.

    -  if you are finding hardboard to be prone to damage have you considered using something more substantial such as MDO (Medium Density Overload)? Because it comes with a resin impregnated paper facing on face and back, there would be no need to seal the back per se.

    - finally, if wanting to really isolate the panel from moisture, take a look at this article we published on our Just Paint site on priming panels for outdoors:

    The alkyd primers used there should give a harder surface and be more moisture resistant.

    Anyway, those are at least some thoughts. And if you like what you are getting, then there is no reason to change.

    Hope that helps.

    Sarah Sands, Senior Technical Specialist, Golden Artist Colors
    2017-11-15 20:28:37
  • ApproveRejectUn-ApproveUser Comment

    ​It does--thanks! I'll give the GAC 200/500 combo a try. 

    2017-11-15 23:03:16
  • ApproveRejectUn-ApproveUser Comment

    ​The other finish that seems to work well is boiled linseed oil (I've specifically used a product called Tru Oil, which is marketed as a finish for gunstocks). Again, this is just for the back and sides of the panel--I know that boiled oils formulated as furniture finishes are going to have a strong tendency to darken, but as a coating for the back side of a panel, this seems like less of an issue. The advantages are that it really soaks into the hardboard (especially on the edges, which helps to strengthen it), it dries fast enough to apply several thin coats in a day, and it doesn't induce any warping of the panel at all (which water-based coatings can sometimes do). 

    It's still not as fast as GAC, though. I can seal the back and sides of a panel with several coats of GAC, and then also put the first coat of oil primer on the front in the same day. 

    2017-11-16 12:26:50

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