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Viewing Topic "Sealing HDF before priming with oil ground"

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Posted By: George O'Hanlon
Total Posts: 2258
Joined Date: Jun 15, 2004

No, I do not think it is a problem to not abrade GAC 100.

George O'Hanlon Technical Director Natural Pigments
Posted : Aug 23, 2017
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015
3. Regardless of what is used for sealing a board (GAC 100, Rublev Fluid Medium, Sealing Varnish), is it alright to apply oil primer directly onto dry layer of sealer, or is it better to lightly sand the dried sealer first (so that it will create a bit of tooth)? That depends on the final surface. If you have a gloss surface, it is best to lightly abrade the surface to reduce the surface to matte.

 

 

I didn't know about that in the past and I primed few panels without sanding the layer of GAC 100. As far as I remember, the surface of dry GAC 100 wasn't really glossy, but it wasn't very matte either. Is this a problem? If oil primer is applied over GAC 100 without sanding the sealer first, can it have negatively affect adhesion? Som of these panels were primed almost two years ago and they don't show any negative effect yet; like cracking, or delaminating... Should I worry? Is there any way to test whether the adhesion of oil primer over the unsanded layer of GAC 100 is sufficient?

Posted : Aug 23, 2017
Posted By: George O'Hanlon
Total Posts: 2258
Joined Date: Jun 15, 2004

1. Is it enough to apply one coat of Sealing Varnish to seal a HDF panel? I'm asking just because for GAC 100 (and other acrylic dispersions) it is recommended to apply two coats. One coat of Sealing Varnish should be sufficient to mitigate support induced discoloration (SID) in acrylic painting. It should also be sufficient to provide some barrier against moisture but more coats will provide a better barrier.

2. After the board is sealed with Sealing Varnish, will it accept only oil priming, or also acrylic? Sealing Varnish will accept both oil and acylic priming.

3. Regardless of what is used for sealing a board (GAC 100, Rublev Fluid Medium, Sealing Varnish), is it alright to apply oil primer directly onto dry layer of sealer, or is it better to lightly sand the dried sealer first (so that it will create a bit of tooth)? That depends on the final surface. If you have a gloss surface, it is best to lightly abrade the surface to reduce the surface to matte.

 

 
George O'Hanlon Technical Director Natural Pigments
Posted : Jul 14, 2017
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015

I have more questions about sealing.

1. Is it enough to apply one coat of Sealing Varnish to seal a HDF panel? I'm asking just because for GAC 100 (and other acrylic dispersions) it is recommended to apply two coats.

2. After the board is sealed with Sealing Varnish, will it accept only oil priming, or also acrylic?

3. Regardless of what is used for sealing a board (GAC 100, Rublev Fluid Medium, Sealing Varnish), is it alright to apply oil primer directly onto dry layer of sealer, or is it better to lightly sand the dried sealer first (so that it will create a bit of tooth)?

 

Posted : Jul 14, 2017
Posted By: George O'Hanlon
Total Posts: 2258
Joined Date: Jun 15, 2004

Dispersive adhesion is a mechanism for adhesion which attributes attractive forces between two materials to interactions between molecules of each material. This mechanism is widely viewed as the most important of the five mechanisms of adhesion due to its presence in every type of adhesive system. The weak interactions that occur between molecules close together taken on an individual basis are not very strong, but when summed over the bulk of a material, can become significant.

The strength of adhesion by the dispersive mechanism depends on the chemical composition of the adhesive system, the degree to which coatings wet each other, and the surface roughness at the interface.

George O'Hanlon Technical Director Natural Pigments
Posted : Jan 31, 2017
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015

The bond between acrylic primer sand oil paint or primers is both dispersive and mechanical adhesion. There shouild be no problem with adhesion to GAC 100 or other acrylic polymers.

 

So, I think I understand mechanical adhesion; basically the oil ground/paint will slightly soak into acrylic layer. However I don't understand, what "dispersion adhesion" is. Can you explain?

Posted : Jan 31, 2017
Posted By: George O'Hanlon
Total Posts: 2258
Joined Date: Jun 15, 2004

The Sealing Varnish does not make a continuous film, because it has a very low resin content—10% weight/volume. It forms slightly absorbent films, which is good for such applications.

The bond between acrylic primer sand oil paint or primers is both dispersive and mechanical adhesion. There shouild be no problem with adhesion to GAC 100 or other acrylic polymers.

George O'Hanlon Technical Director Natural Pigments
Posted : Jan 5, 2017
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015

The advantage of the Sealing Varnish over water-based coatings is that it does not encourage fiber raising and swelling. Otherwise, GAC 100 is also a good cboice.

I applied one layer of Rublev Oil Ground on HDF sealed with Sealing varnish (second layer will be applied today). Prior to sealing, I cleaned the surface with Isopropanol and then applied one layer of Sealing Varnish without sanding the board, just as you recommended.

However, as I mentioned earlier, before I bought Sealing Varnish I used to lightly sand hardboard and then seal it with two coats of GAC 100 (I gave it at least a week to dry thoroughly). If I'm to compare applying oil ground over GAC 100 and over Sealing varnish, the layer of ground applied over Sealing Varnish seems to dry to more matt surface; maybe sealing with varnish left board sligthy absorbent?

That made me wonder about GAC 100; how good is adhesion of oil ground applied over a surface sealed with e.g. two coats of GAC 100 (or similar acrylic dispersion)? I understand that the bond between acrylic primer and oil color is purely mechanical, therefore good acrylic primers contain mineral extenders (like marble dust, talc) to give the primer absorbency and tooth. However GAC 100 is just the pure polymer. Should I be concerned about the longevity of panels which I prepared in past?

Posted : Jan 5, 2017
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015

I should've said to apply the Sealing Varnish thinly as in a thin coat.

High-density fiberboard and hardboard is denser and much stronger and harder than medium-density fiberboard and particle board because it is made out of exploded wood fibers that have been highly compressed. Consequently, the density of hardboard is 31 lbs or more per cubic foot (500 kg/m³) and is usually about 50-65 lbs per cubic foot (800–1040 kg/m³). It differs from particle board in that the bonding of the wood fibers requires no additional materials, although resin is often added. The hardboard variety is made without resin. The HDF version is made with resin.

Tempered hardboard is hardboard that has been coated with a thin film of linseed oil and then baked; this gives it more water resistance, impact resistance, hardness, rigidity and tensile strength. An earlier tempering process involved immersing the board in linseed oil or tung oil until it was 5 to 6 percent saturated, and heating to 170 °C (340 °F). The use of linseed oil has greatly diminshed and has been replaced with other coatings.

Most hardboards have a protective surface of some sort, but those that are tempered especially do. The smooth side has the surface, which is not neessarily a coating but formed during manufacture. There are a standards for hardboard, one of which can be seen here: 
Composite Panel Organization.

The surface of hardboard has enough texture for good adhesion. The reason for not recommending sanding is that it removes the surface that has limited moisture absorption and thereby increases fiber wetting, raising and swelling.

The advantage of the Sealing Varnish over water-based coatings is that it does not encourage fiber raising and swelling. Otherwise, GAC 100 is also a good cboice.


Aha, so I can apply it undiluted, but it is necessary that it is a thin coat. And I shouldn't sand it regardless if there is coating or not. Right?

 

Posted : Nov 29, 2016
Posted By: George O'Hanlon
Total Posts: 2258
Joined Date: Jun 15, 2004

I should've said to apply the Sealing Varnish thinly as in a thin coat.

High-density fiberboard and hardboard is denser and much stronger and harder than medium-density fiberboard and particle board because it is made out of exploded wood fibers that have been highly compressed. Consequently, the density of hardboard is 31 lbs or more per cubic foot (500 kg/m³) and is usually about 50-65 lbs per cubic foot (800–1040 kg/m³). It differs from particle board in that the bonding of the wood fibers requires no additional materials, although resin is often added. The hardboard variety is made without resin. The HDF version is made with resin.

Tempered hardboard is hardboard that has been coated with a thin film of linseed oil and then baked; this gives it more water resistance, impact resistance, hardness, rigidity and tensile strength. An earlier tempering process involved immersing the board in linseed oil or tung oil until it was 5 to 6 percent saturated, and heating to 170 °C (340 °F). The use of linseed oil has greatly diminshed and has been replaced with other coatings.

Most hardboards have a protective surface of some sort, but those that are tempered especially do. The smooth side has the surface, which is not neessarily a coating but formed during manufacture. There are a standards for hardboard, one of which can be seen here: 
Composite Panel Organization.

The surface of hardboard has enough texture for good adhesion. The reason for not recommending sanding is that it removes the surface that has limited moisture absorption and thereby increases fiber wetting, raising and swelling.

The advantage of the Sealing Varnish over water-based coatings is that it does not encourage fiber raising and swelling. Otherwise, GAC 100 is also a good cboice.

George O'Hanlon Technical Director Natural Pigments
Posted : Nov 29, 2016
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015

1. Sealing Varnish does not need to be diluted to apply by brush, and for your application it is necessary to thin it.

Does not need to be diluted to apply by brush, and for my application it is necessary to thin it... Eh, maybe I'm not getting something, but to me these two parts of your statement are contradicting; I will apply it by brush, so do I have to thin it or not?

If I have to thin it to use as a sealer, what is recommended amount of alcohol? The description says up to 20 %, which means, that it can be also less.

One more note, we do not recommend sanding the surface of hardboard, because it already has a protective finish that is easily removed by sanding it. Simply clean it with isopropyl alcohol before applying any coating.

HDF already has protective finish? I didn't know that. Just to be sure; I don't know whether there are any international "standards" for how hordboard should be made and look; HDF that I buy look like this:

http://photos.tradeholding.com/attach/hash60/162876/hardboard__2_.jpg

There is a rough textured side and a smooth side.

http://europlywood.com/wp-content/uploads/2009/11/IMG_5303.JPG

http://europlywood.com/wp-content/uploads/2009/11/IMG_5304.JPG

So this smooth side has the coating you mentioned, right?

 

The reason why I sanded it was to create a slightly rough surface. I thought it would provide better adhesion for whatever would be applied on it (GAC 100, Sealing Varnish, Acrylic primer). But after reading what you wrote, I take it, that the adhesion would be sufficient even on smooth surface, is that right?

 

Last thing; as I mentioned in first post, currently I seal HDF using GAC 100. So, using Sealing varnish is another option. What do you think, which of these two materils is better to seal? Are they equally good, or is varnish better option (in terms of longevity and adhesion of oil gorund onto it)? If both GAC 100 or Sealing varnish are given enough time to dry properly, is there any risk of delamination of oil ground from these sealers?

Posted : Nov 29, 2016
Posted By: George O'Hanlon
Total Posts: 2258
Joined Date: Jun 15, 2004

1. Sealing Varnish does not need to be diluted to apply by brush, and for your application it is necessary to thin it.

2. Apply only one coat of Sealing Varnish and apply it thinly.

3. Allow Sealing Varnish to dry overnight before priming with oil ground.

One more note, we do not recommend sanding the surface of hardboard, because it already has a protective finish that is easily removed by sanding it. Simply clean it with isopropyl alcohol before applying any coating.

George O'Hanlon Technical Director Natural Pigments
Posted : Nov 28, 2016
Posted By: enthusiast
Total Posts: 69
Joined Date: Mar 13, 2015

When I prepare hardboard (HDF) for oil painting, I lightly sand and seal the surface first, before applying oil ground. To seal it, I apply two layers of Golden GAC 100. I'm a bit concerned about adhesion of oil ground onto GAC 100. So far it seems to work well, but still, I've been wondering, if there is an alternative, something more "compatible" with oil ground. Is it alright to use Sealing Varnish? Or maybe Shellac? Which of these two is better?

Hmmm, Sealing Varnish looks promising, according to the description. I have some questions about using it:

1. According to the description, it can be diluted with etanol or denatured alcohol. The question is: should it be diluted beforte applying with the brush? Is there difference in performance between concentrated and diluted varnish?

2. I suppose, that whether concentrated or diluted, the layer has to be thin. How many layers of Sealing Varnish need to be applied?

3. When using GAC 100 I apply a coat, then after cca. 1,5 hour second coat and then let it dry for a week. As for Sealing Varnish, after it is applied and after it has dried, how long is it necessary to wait before priming with oil ground?

Posted : Nov 28, 2016

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