How to avoid Surface Tension problems with woodwork finish?

[Jmanm]

I have been having a hard time brushing on a 2-part polyurethane woodwork finish clear. First off it is nothing like waterbased or oil based polyurethane clears. The product I'm using is plagued by surface tension problems & no matter what I do I can never get a consistent brush result. I have ruled out many things like: different styles of brushes, range of temp, brushing on thick or thin & even thinning with a supplied solvent up to 30-40%.

 

First example: on flat surface that is level with sharp 90 degree edges, the clear always moves inwards from the edges a few mm thinning right near the edge. Also in this example I always get many pinholes (air that was trapped in the coat & escaped creating a defect)

 

 

However on a highly curved surface like a 2inch wide dowel, I get virtually no pinholes but I do get alot of dry patches forming (they look like areas you missed with the brush).

 

So it is very hard to get consistent results let alone finish a product. No searching in typical woodwork community is of any help as I think most finishers just spray this stuff. So any help is appreciated.

 

Thanks

 

 

 

 

Edited October 25, 2016 by Jmanm

[DrP]

Try adding a surface tension reducing surfactant or solvent. I have found that Butyl Glycol to be one of the most effective (is harmful, but not too bad if you are using just once).. Just add somewhere between 0.2 to 1 % depending on how bad it is and it will wet out much better. I am sure there are other surface tension reducing solutions you can try depending on what is available to you.

[Jmanm]

Thanks.

 

Do you know roughly how long until that tiny % of 0.2-1 will fully mix with the product?

[DrP]

Depends how you mix it in - if you mix it in with a drill then about 10 seconds should be ample. If you are using one of those McDonnalds coffee stirrers then it will take a lot longer. I'd say mix it in with a drill for 10 - 20 seconds or use a paddle and mix for a minute or 2.

[Jmanm]

Depends how you mix it in - if you mix it in with a drill then about 10 seconds should be ample. If you are using one of those McDonnalds coffee stirrers then it will take a lot longer. I'd say mix it in with a drill for 10 - 20 seconds or use a paddle and mix for a minute or 2.

Thanks, reason I asked is that seemed like such a small % to be mixed evenly in a large body even if it was water.

 

 

I also have some further questions hopefully you or anyone else could answer.

 

Does high humidity worsen the surface tension problems or have no real effect?

 

Since I'm only getting pinholes on flat level surfaces & virtually none on curved that are vertical. How could surface tension cause them if it does.

[DrP]

There are various causes of pinholing. Is the surface clean? Sometimes impurities, a few molecules of oil or dirt, can cause pinholing. Wiping the surface with a solvent can help, but this can also cause problems if there is residual solvent molecules on the surface to be treated as they too can cause pin holes. Make sure the surface is ultra clean and give it time to dry completely if wiped with a spirit. Maybe you need a primer to aid adhesion - what is the substrate?

 

Question: Does the 2 pk PU thicken when it has been mixed? Can you wait for the viscosity to build a bit before application. If you have a way to increase the viscosity too then that might help. Also, if it continues then try adding a bit more of whatever you are using as a surface tension reducer.

 

So - Reduce surface tension. Let the viscosity build. Clean surfaces and ensure no residual solvent on substrate. Consider a primer if you still have problems.

 

Further - you could contact the paint supplier and tell then about their paint - they might have suggestions or they might consider altering their product to contain more or better rheology aids and surfactants.

[DrP]

PS - Regarding your humidity question - high humidity can slow the drying times of waterbased paints considerably. So does low temperature. Whether this can effect the pinholing problem or not I am not so sure I am afraid. Anything that messes with the smooth consistent drying of a coating is undesirable, but as I said, I think this will only effect your drying times and probably wont impact on the pinholing problem.

[John Cuthber]

One component of a two part polyurethene will be a diisocyanate. This will react with the glycol.

That may or may not make a difference to the finished product.

It will also react with any water present in the timber or from the air.

The other component will probably be a glycol or other polyhydroxy alcohol, so adding a small amount of a glycol might not make much difference to the surface tension etc of the product.

[StringJunky]

I don't know if this is relevant that I learnt from reading stuff by luthiers, is that those pinholes might be 'fisheyes' which, they say, is caused by silicone spots on the wood prior to spraying the lacquer. If you use silicone polish or such normally then you need to be fastidious about it not contaminating the wood prior to spraying. You can get an additive that prevents fish-eyes - it's actually pure silicone apparently. I copied this response by a woodworker:

 

 

Get some Fish Eye Eliminator..or FEE..an automotive paint store, Homestead Finishing, etc will have it. A small bottle should last a lifetime.

Fish eyes are generally caused by wax or similar silicone on the surface or in the finish that you're finishing over...that little spot of wax breaks the surface tension in the spot..and the finish doesn't flow evenly.

FEE is just a high concentration of liquid silicone. Put the required amount of FEE in your new finish (usually a squirt or so) and that evens out the surface tension.

Of course all future finishes applied on top of yours will also require FEE.

http://forums.finewoodworking.com/fine-woodworking-knots/finishing/correcting-fisheyes-lacquer

Note: It is possible to carry silicone on your hands from home onto your work; it is in a lot of products and its source may not be immediately obvious.

Edited November 3, 2016 by StringJunky

[Jmanm]

There are various causes of pinholing. Is the surface clean? Sometimes impurities, a few molecules of oil or dirt, can cause pinholing. Wiping the surface with a solvent can help, but this can also cause problems if there is residual solvent molecules on the surface to be treated as they too can cause pin holes. Make sure the surface is ultra clean and give it time to dry completely if wiped with a spirit. Maybe you need a primer to aid adhesion - what is the substrate?

 

Question: Does the 2 pk PU thicken when it has been mixed? Can you wait for the viscosity to build a bit before application. If you have a way to increase the viscosity too then that might help. Also, if it continues then try adding a bit more of whatever you are using as a surface tension reducer.

 

So - Reduce surface tension. Let the viscosity build. Clean surfaces and ensure no residual solvent on substrate. Consider a primer if you still have problems.

 

Further - you could contact the paint supplier and tell then about their paint - they might have suggestions or they might consider altering their product to contain more or better rheology aids and surfactants.

Hi.

 

I can say 100% the surface is clean & these pinholes are not caused by contamination. I have tried a few cleaners too, alcohol, wax & grease remover, thinners, just water & detergent & make sure its fully dried. Besides these pinholes don't happen on curved vertical placed surfaces, only flat wood lying level.

 

The substrate is typical hardwood with a few coats of PolyU under it, so technically just a subcoat of sanded polyu.

 

The polyU doe's thicken a bit once both parts A & B are mixed. But still resonable thin & flowing.

 

The manufacturer's only suggestion is to thin to avoid pinholes, but I have done 5%, 10, 15,20 & down to 35% which is far further then their 10% max recommendation. Still always get pinholes.

Edited November 8, 2016 by Jmanm

[StringJunky]

Hi.

 

I can say 100% the surface is clean & these pinholes are not caused by contamination. I have tried a few cleaners too, alcohol, wax & grease remover, thinners, just water & detergent & make sure its fully dried. Besides these pinholes don't happen on curved vertical placed surfaces, only flat wood lying level.

 

The substrate is typical hardwood with a few coats of PolyU under it, so technically just a subcoat of sanded polyu.

 

The polyU doe's thicken a bit once both parts A & B are mixed. But still resonable thin & flowing.

 

The manufacturer's only suggestion is to thin to avoid pinholes, but I have done 5%, 10, 15,20 & down to 35% which is far further then their 10% max recommendation. Still always get pinholes.

If they are not occurring on vertical surfaces then that suggests to me something is landing either on the prepared surface prior to brushing on or after brushing on.the lacquer. It may be coming off your clothes or from the air over the work surface via air draughts..

Edited November 8, 2016 by StringJunky

[Jmanm]

If they are not occurring on vertical surfaces then that suggests to me something is landing either on the prepared surface prior to brushing on or after brushing on.the lacquer. It may be coming off your clothes or from the air over the work surface via air draughts..

Hi. I ruled this out as I stored the piece in a small dust free environment, besides the pinholes are clean & caused by trapped air. The defect goes down pretty deep into the coat something debris doesn't do.

 

My thinking is they are caused by a combination of the brush bristles, & something at the molecular level that happens when the piece is flat/level. also from the surface tension & gravity maybe.

 

I bet if i sprayed this stuff the pinholes wouldn't happen.

[StringJunky]

Hi. I ruled this out as I stored the piece in a small dust free environment, besides the pinholes are clean & caused by trapped air. The defect goes down pretty deep into the coat something debris doesn't do.

 

My thinking is they are caused by a combination of the brush bristles, & something at the molecular level that happens when the piece is flat/level. also from the surface tension & gravity maybe.

 

I bet if i sprayed this stuff the pinholes wouldn't happen.

Right,OK. Just going through the list.

Edited November 10, 2016 by StringJunky

[DrP]

Yea, sorry - only thing left I can think of is to add even more surfactant then.

[StringJunky]

Yea, sorry - only thing left I can think of is to add even more surfactant then.

Is the liquid too viscous for the method used and surfactant sorts that?

 

Inspired by this article:

 

http://crosslinktech.com/support/tips-and-tricks/degassing-materials.html

 

Avoid introducing air bubbles when mixing - think about your mixing technique. Add surfactant like DrP says.

 

Failing these, you could try using a pot that has a buit in vacuum pump to degass the lacquer after mixing. Here's one used in fishing for soaking pellets in. These pots enable the pellets to sink, instead of floating, by sucking the air out of them. I'm sure you could find them used for other applications, like food preservation, but this should give you an idea what to look for or even inspire you to create your own using the same idea.

 

http://www.pecheur.com/en/gb/buy-pellet-pump-garbolino-87483.html?af=393329&gclid=CI262vT2ndACFYsA0wodPc8DMw#af=393329

 

 

Edited November 10, 2016 by StringJunky

[DrP]

QUOTE SJ: "Is...does the surfactant sort this out"

 

The surfactant will reduce surface tension further and it is more likely to wet out better... pinholes will be more likely to just seal over under the weight of the coating wetting on the surface.

 

@JMANN

You can get anti foaming additives/surfactants which pop bubbles too if you think it is trapped air..

 

Just a note on the surface cleaning - if a surface is wiped with solvent to get grease off, it can take a surprisingly long time for EVERY molecule of the solvent to leave the surface... long after it looks, feels and smells completely dry... just a few solvent molecules trying to escape through a waterbased film can cause pinhole... (NOT saying that this IS the case... just saying that it is complicated and the reasons are varied) - if you wiped the substrate with spirits at all then leave it a few hours before application or maybe flash the surface with a flame. However - I still reckon that the right amount of a suitable surfactant will solve it. - Good luck.