Why is my varnish so thick?

[ExFabrica]

Hi everyone,

 

I've been working on making some traditional  oil varnish for the past couple of weeks and, of course, my results have generated some questions.

I was following a simple recipe, one that I was tought, and have seen cited a couple times on this site and elsewhere. It goes like this:

 

• 1 part colophony (cooked to the desired degree);
• 1 part boiled linseed oil, purified;
• An amount of mastic equal to 5% of the mixture's total weight. 

 

For my first batch, I cooked the colophony for just under 10 hours. I don't know what temperature I was cooking at (afterwards, I realized my thermometer wasn't cut out for such temperatures; I was going in blind). I assume it was pretty hot, since the colophany reduced by about 75%, having a starting weight of 180g and finishing at 30g. I'm pretty sure I didn't burn the resin. It kept a somewhat familiar texure despite being brittle, and it remained soluble in linseed oil. I was told burnt colophany no longer disolves in linseed oil, but that's all I have to work with and I'm not sure if that's true.

With the resin in fusion, I added 30g of linseed oil and kept cooking for an hour until a drop of varnish on a glass plate could be stretched about 10-15 cm. I then let the mixture cool a bit and added 3g of mastic.

I didn't add any trups to the mix.

When I went to can the varnish and let it cool, my varnish became incredibly thick. Barely softer than colophony. 

 

I'm wondering what could have caused this intense thickness? 

Dit I overcook the resin?

Are my ratios off?

Should I have added turpentine before finishing the cook?

 

After the first batch, I made two more. This time, I had a good thermometer that allowed me to make sure I never cooked over 225°C. 

I made one batch with colophony cooked for 6 hours, reduced by about 23% from it's starting weight. A second one cooked for about 12 hours, reduced by approximately 45% of it's starting weight if my calculations are correct.

Though these 2 subsequent batches are somewhat less viscous than the first, they are still very thick and would be difficult to apply without at least heating or thinning them.

 

I've heard varnish can be diluted with linseed oil. Is this true?

Does it need to be heated or even cooked for this to work?

I assume this would affect the oil to resin ratio, making the varnish "fatter".

How about diluting with turpentine? Could that be a solution to thinning the varnish without altering it's drying properties too much?

 

Thanks for any help or advice on this. I know it's had to find precise answers without knowing the precise process, but hopefully I can shed some light on any details I may have left out. I will try and add pictures that could help give an idea tomorrow.

[Jim Bress]

The first part (as I was taught to make this varnish) is your varnish, and it's essentially clear. I melt the rosin, then add the linseed oil at 200 C for 2 hours using the proportions you show.

Your first batch is the color that you add to the clear varnish. About 50% volume as the clear varnish. Then proceed with the mastic and adjust viscosity as you incorporate the mastic by adding more linseed oil as needed.

[Sean Couch]

I have made that varnish three times. The first time I made some clear and some colored varnish. For the colored varnish I cooked the resin for 168 hours at 350F. It had a 24% reduction and after mixed 1:1 with linseed oil and the mastic, I needed to thin it with turpentine. 3 parts varnish to 1 part turp was what I used to make it brushable.

The 24% reduction resin didn’t have much color so I cooked the remaining rosin and got an overall reduction of 52.5%. I did the same 1:1 resin to oil with 10% mastic and 27% turpentine. When cooled, it was so thick that it wouldn’t even pour out of the bottle I put it in.

The third time I used 1:1 oil/resin, 5% mastic, and 42% turpentine. When cooled, it was still a bit thick, but it was pourable. I added a bit more turp to make it brushable.

For the first cook, I did not add any turp during the cook. For the 2nd and 3rd time I did add turp to the cook. I used the first cook on violin #1 and the 3rd cook on violin #2. An interesting finding is that the weight of each layer of varnish was about twice as heavy on #2 as it was on #1. Either my layers were laid on thicker on #2, or the varnish is denser when it is made from highly reduced colophony.

All three batches of varnish had no problem curing without any drying agent added, but It does seem to be a bit soft. I don’t really know how hard it is supposed to be, so take my words with a grain of salt. Last night, on violin #2, I did the initial string up and I could immediately see the varnish getting damaged under the bridge feet.

After applying a coat of varnish, I pad it out with my fingertips and let it sit for a few hours until the turpentine evaporates. Then it goes into the UV box. After about 4 hours it is dry to the touch, but needs 30 hours to pass the fingerprint test.

[Steve Voigt]

15 hours ago, ExFabrica said:

Hi everyone,

 

I've been working on making some traditional  oil varnish for the past couple of weeks and, of course, my results have generated some questions.

I was following a simple recipe, one that I was tought, and have seen cited a couple times on this site and elsewhere. It goes like this:

 

• 1 part colophony (cooked to the desired degree);
• 1 part boiled linseed oil, purified;
• An amount of mastic equal to 5% of the mixture's total weight. 

 

For my first batch, I cooked the colophony for just under 10 hours. I don't know what temperature I was cooking at (afterwards, I realized my thermometer wasn't cut out for such temperatures; I was going in blind). I assume it was pretty hot, since the colophany reduced by about 75%, having a starting weight of 180g and finishing at 30g. I'm pretty sure I didn't burn the resin. It kept a somewhat familiar texure despite being brittle, and it remained soluble in linseed oil. I was told burnt colophany no longer disolves in linseed oil, but that's all I have to work with and I'm not sure if that's true.

With the resin in fusion, I added 30g of linseed oil and kept cooking for an hour until a drop of varnish on a glass plate could be stretched about 10-15 cm. I then let the mixture cool a bit and added 3g of mastic.

I didn't add any trups to the mix.

When I went to can the varnish and let it cool, my varnish became incredibly thick. Barely softer than colophony. 

 

I'm wondering what could have caused this intense thickness? 

Dit I overcook the resin?

Are my ratios off?

Should I have added turpentine before finishing the cook?

 

After the first batch, I made two more. This time, I had a good thermometer that allowed me to make sure I never cooked over 225°C. 

I made one batch with colophony cooked for 6 hours, reduced by about 23% from it's starting weight. A second one cooked for about 12 hours, reduced by approximately 45% of it's starting weight if my calculations are correct.

Though these 2 subsequent batches are somewhat less viscous than the first, they are still very thick and would be difficult to apply without at least heating or thinning them.

 

I've heard varnish can be diluted with linseed oil. Is this true?

Does it need to be heated or even cooked for this to work?

I assume this would affect the oil to resin ratio, making the varnish "fatter".

How about diluting with turpentine? Could that be a solution to thinning the varnish without altering it's drying properties too much?

 

Thanks for any help or advice on this. I know it's had to find precise answers without knowing the precise process, but hopefully I can shed some light on any details I may have left out. I will try and add pictures that could help give an idea tomorrow.

First of all, it sounds like you did almost everything right on your first try. Congratulations. The only thing you are missing is turpentine. 

I would expect the varnish you made to be almost solid at room temperature. The harder you cook the colophony, the thicker the final varnish will be, and the more turps you will need to add. 

I recommend taking the oil/resin mix off the hot plate and adding the turps while the mixture is hot, but not so hot as to cause the turps to boil. To be safe on your first try, add the turps at 150 C. Later, you can experiment with adding it hotter, if your risk tolerance allows. If you do that, make sure you have the cover handy—if turps starts to boil, putting the cover on will cut off oxygen. 

You can also use more oil, but then you are making a different varnish. It will have longer open time, but it will be softer and more flexible. 

The degree to which you process the oil also matters. I will guess that you used raw oil. If you washed it and heated it, your varnish would be even thicker, and you wouldn’t need to cook it for so long. 

You’re off to a good start—good luck and let us know how it goes. 

[David Burgess]

17 hours ago, ExFabrica said:

I've heard varnish can be diluted with linseed oil. Is this true?

Yes, varnish can be diluted with linseed oil, but that's not the same as diluting with a solvent which evaporates completely. The linseed oil will remain and change the properties of the varnish.

1 hour ago, Steve Voigt said:

How about diluting with turpentine? Could that be a solution to thinning the varnish without altering it's drying properties too much?

Turpentine is a bit of a wild card, since some undetermined portion will evaporate completely, while some other undetermined portion will already be a rather unstable resin, with other parts becoming a similarly unstable resin with exposure to air and light.

Products called turpentine can vary widely, depending on things like the methods of extraction and other processing, the source, the age, and the amount of exposure to air and light.

[Steve Voigt]

54 minutes ago, David Burgess said:

 

Turpentine is a bit of a wild card, since some undetermined portion will evaporate completely, while some other undetermined portion will already be a rather unstable resin, with other parts becoming a similarly unstable resin with exposure to air and light.

Products called turpentine can vary widely, depending on things like the methods of extraction and other processing, the source, the age, and the amount of exposure to air and light.

Dave, 

I’d certainly like to hear more of your thoughts on this. What makes you say that turpentine resin is unstable? Also, what solvent(s) do you use?

My experience: A drop of reasonably fresh turpentine on glass will completely evaporate. A drop that has been aged and has changed color will leave behind a resin that dries hard and appears stable to me. I’d love to know what has convinced you otherwise. 

[Don Noon]

Turpentine can vary depending on the source; it's a mixture of a bunch of organic stuff from a tree.  Testing a drop on glass is to me a necessary step.

I like to use lavender oil, as it evaporates very slowly.  A bit of turpentine too, as I want the varnish to get out of the runny stage fairly quickly, but then spend a while in the leveling stage.  But again, I had one source of lavender oil the NEVER dried on glass.

I have made many batches of 1:1 rosin/linseed varnish, and they were all way too thick to use without thinning.

[ExFabrica]

Thanks for all the replies, it's keeping me on the right track.

20 hours ago, Sean Couch said:

For the first cook, I did not add any turp during the cook. For the 2nd and 3rd time I did add turp to the cook. I used the first cook on violin #1 and the 3rd cook on violin #2.

I didn't add turps to any of my cooks, but seeing the results and going off the feedback I've gotten so far, I think it would be a good idea to try it next time. One pleasant surprise I've found is that I can add turps afterwards and the varnish dilutes perfectly into it. I've also found that I need a really small amount (so small I have a hard time measuring it by weight) of it to get a good consistency. I heat my varnish and turps in a double boiler and they combine right away.

From my limited testing, I feel like a certain amount of turps is necessary to get a workable consistency and to break the "gummy-ness" of the varnish.

19 hours ago, Steve Voigt said:

I would expect the varnish you made to be almost solid at room temperature. The harder you cook the colophony, the thicker the final varnish will be, and the more turps you will need to add. 

I recommend taking the oil/resin mix off the hot plate and adding the turps while the mixture is hot, but not so hot as to cause the turps to boil. To be safe on your first try, add the turps at 150 C. Later, you can experiment with adding it hotter, if your risk tolerance allows. If you do that, make sure you have the cover handy—if turps starts to boil, putting the cover on will cut off oxygen. 

You can also use more oil, but then you are making a different varnish. It will have longer open time, but it will be softer and more flexible.

That's right, my darkest varnish, the one using resin that was cooked for the longest time and at a higher temperature, is basically unworkable at room temp. Even when heated, it needs to be thinned. Fortunately, it also has a really nice colour when prepared for application.

I'm curious to know what would be the effect of heating turpentine at higher temperatures during the cook? I'll have to try it out next time.

One thing that is still confusing to me is the concept of Fat Over Lean. It's sounds really straightforward, does it simply mean that varnishes with a higher oil content should be applied later in the process? Steve, I gather this is what you mean when you say changing the oil ratio will ultimately produce a different varnish. If I'm not mistaken from what I've read on varnish, it could be considered as a kind of "spectrum" depending on the ratio of oil-to-resin.

How does turpentine play into this? I would guess it makes a leaner varnish.

Is there a considerable difference when adding oil at hot temperatures (during the cook) vs. adding it cold after the cook?

 

I know these are rudimentary questions that have been asked here before, but the answers are still kind of hazy for me. Thanks to anyone who wants to chime in to help me clear it up.

 

I'm doing application tests today. I'll share some pictures when it's dry enough.

[Deo Lawson]

Add turpentine until it's good. In my opinion, it's not rocket science. I haven't had any problems with turp.

I did ruin a batch of amber varnish by trying to "dilute" with linseed oil back in the day. I got a varnish that never fully hardened and always took prints, for example from chinrest hardware or neck wall hangers. Theoretically I could un-ruin it by cooking again, and adding more resin, but that's mustard after dinner, as we say in Poland.

[Urban Luthier]

On 10/20/2025 at 11:19 PM, ExFabrica said:

• 1 part colophony (cooked to the desired degree);
• 1 part boiled linseed oil, purified;

Before reading any further - did you measure the ingredients by weight or did you measure grams for the colophony and ml for oil? if you did the latter you may well come out with a varnish that is near solid depending on how long you cooked it for. 

[Wood Butcher]

1 hour ago, Urban Luthier said:

Before reading any further - did you measure the ingredients by weight or did you measure grams for the colophony and ml for oil? if you did the latter you may well come out with a varnish that is near solid depending on how long you cooked it for. 

In the first post, they mention once cooked, 30g of rosin was left (from 180g), to which, 30g of linseed oil was then added and cooked further. Finally, adding 3g of mastic.

[ExFabrica]

On 10/24/2025 at 1:51 PM, Urban Luthier said:

Before reading any further - did you measure the ingredients by weight or did you measure grams for the colophony and ml for oil? if you did the latter you may well come out with a varnish that is near solid depending on how long you cooked it for. 

Yes, like the poster above mentionned, I did go by weight, so 30g of resin for 30g of oil.

 

Below is a picture of my 2nd batch, also my biggest. I'm not sure what went wrong with it, but it's cloudy, kind of milky and almost looks chocolatey in the jar. It also has a weird texture, more like fudge, and it won't run at all.

 

Has anyone ever encountered something like this? I'm not sure what could have gone wrong.

 

Edited October 27 by ExFabrica

[John Harte]

On 10/28/2025 at 11:47 AM, ExFabrica said:

Below is a picture of my 2nd batch, also my biggest. I'm not sure what went wrong with it, but it's cloudy, kind of milky and almost looks chocolatey in the jar. It also has a weird texture, more like fudge, and it won't run at all.

Has anyone ever encountered something like this? I'm not sure what could have gone wrong.

I have occasionally got this result.

Sometimes it seems due to the cooked resin and oil not having combined as well as you would want. If this is the case, a small amount of heat under an IR lamp may be enough to get rid of the cloudy look. However it is generally better to try heating the varnish back up to a good cooking temperature, giving it more time at that temperature. If you do try this and have already added turpentine, be very careful and patient as you approach the boiling point of the turpentine. Without proper combining of the various components, these may separate out in the jar over time, at least in my experience.

It could be that the colophony you are using has low resin acid content. Some years ago I had several attempts at making varnishes using Norway spruce callus resin. The end result was similar to what you have produced. I subsequently discovered that Norway spruce callus resin has particularly low resin acid content and have since wondered whether this was behind my lack of success. If my best guess regarding this is correct, in your case low resin acid content could be the result of some sort of treatment involved in producing the original colophony or you cooking the colophony at a too high temperature or the original resin merely having low resin acid content to start with.

Is there any turpentine involved in your latest batch? If so, I would be looking at that as well.

I also would think about the oil you are using. In your case, what exactly is boiled linseed oil, purified?

Finding a good reliable resin source, oil and its processing that works and a good source of turpentine are not trivial. And then there is the establishing of a cooking process that reliably works, part of which comes from getting to know how each of these components behaves individually and together over ranges of temperature and timings.

While this latest batch may seem to be a failure, it potentially is a huge opportunity to learn far more than an apparent successful result might provide. Varnishes that initially seem okay can over time turn out to be anything but...

[Tets Kimura]

I use a few different varnishes, and one of them is a 50:50 mix of linseed oil and cooked colophony. I don’t thin it with turpentine or any other solvent. It’s thick like honey, but not sticky, and I’ve never had any trouble applying it without heating.

One common — and widely misunderstood — aspect of varnish cooking is the idea that the longer you cook the oil and resin together, the better they will combine. In reality, the more you heat the oil, the more its quality deteriorates.
If the OP’s oil had already been heavily processed before it was combined with the resin, I’d say that’s one of the issues.

[ExFabrica]

21 hours ago, John Harte said:

Is there any turpentine involved in your latest batch? If so, I would be looking at that as well.

I also would think about the oil you are using. In your case, what exactly is boiled linseed oil, purified?

Finding a good reliable resin source, oil and its processing that works and a good source of turpentine are not trivial. And then there is the establishing of a cooking process that reliably works, part of which comes from getting to know how each of these components behaves individually and together over ranges of temperature and timings.

While this latest batch may seem to be a failure, it potentially is a huge opportunity to learn far more than an apparent successful result might provide. Varnishes that initially seem okay can over time turn out to be anything but...

Thank you for the thoughtful response. I'm not too dissapointed in how this batch turned out, more so in the fact that winter is right around the corner and it will be hard for me to cook more batches in the coming months. I'm excited to cook some more when summer comes around next year!

17 hours ago, Tets Kimura said:

One common — and widely misunderstood — aspect of varnish cooking is the idea that the longer you cook the oil and resin together, the better they will combine. In reality, the more you heat the oil, the more its quality deteriorates.
If the OP’s oil had already been heavily processed before it was combined with the resin, I’d say that’s one of the issues.

Yes, I get the sense from many of the responses I've gotten that the base materials are very important. Here is exactly the products I used, in case anyone has any experience with them and could offer some feedback.

Resin was:

Colophony:

https://www.kamapigment.com/produits-kama/resines-colles-gommes/colophane.html

Mastic:

https://www.kamapigment.com/produits-kama/resines-colles-gommes/resine-mastic-en-larmes.html

Linseed oil was from Detwiller linseed, which I know some people on here have used for making varnish. High quality as far as I can tell. It is sold by an other company now, but I'm pretty it's the same as this:

https://kingsfleet.ca/products/kingsfleet™-boiled-linseed-oil?variant=45519596192055

I washed it by mixing it with water until the water, once separated from the oil, was clear. As far as I know, that's the process of purification.

I also used hardware store turpentine.

 

And that's all I used. Like I mentionned in the original post, I didn't add turpentine for the cook. I did add some afterwards, to thin the varnishes and to facilitate filtration.

I don't think the turps or even the ingredients were the culprits, though I may be mistaken. I got two good, clear batches using the same ingredients. The difference between batches was how long I cooked the resin.

 

One thing I suspect may have been a problem is water getting into the mix. For this second batch (the one that is cloudy) I had a bit of residual water in my linseed oil (from washing). I tried to boil it out, but I think there may have been a few mL left in the oil when I added it to the resin. 

An other working theory I have is that I may have cooked it when it was a bit too cold outside? The sun had gone down and the temperature dropped quickly, to about 13 degrees C. It made doing the "string test" really difficult.

 

Anyways, I'm just adding these details here for posterity. Thanks to everyone for the help so far, and if you have anything to add, I'd be glad to hear it!

[David Burgess]

On 10/23/2025 at 9:37 PM, Deo Lawson said:

Add turpentine until it's good. In my opinion, it's not rocket science. I haven't had any problems with turp.

If one is cooking their resin to darken it, I wouldn't recommend that. The addition of fresh turpentine, or too much turpentine can cause the ingredients to precipitate out or separate.

Whether or not this separated varnish might still be viable, I do not know, since I haven't tried it.