Bubbles in NC lacquer

I am getting some grief from bubbles forming in my NC lacquer.

The bubbles don't form where the grain is fine and tight, but rather where the grain is open and usually darker. Also I get the bubbles almost exclusively on horizontal surfaces. The bubbles run along the grain.

When the sanding sealer is on, I can't see any defects. Once the first coat of lacquer goes on, I can see tiny pin holes. Usually, it is the second coat of lacquer that seals over these pin holes, and then bubbles form.

I tried sealing the surface with a thinned out coat to begin with, but that has not helped.

Anyone able to help me understand both cause and cure?

I put a post up on the same issue several months ago and got some very good replies from Soundman and others. The consensus seemed to be that the air is driven hard into the pores by the spraygun, then a layer of lacquer is laid down, and the air is still finding its way out. It could also be that temperature changes caused by the act of compressing air has an effect - in other words the air is chilled, then when it warms up again it expands and needs to find a way out.
I still get it occuring though. I think I've reduced the problem by:
1. Spraying two coats of sealer. Its not realy spraying to spec but it seems to work to a limited degree.
2. Backing off the air pressure when I think its going to happen. Having a Lvlp gun I can get it to atomise at a very low pressure, just go slow for a while like I were using a touch up gun.
3. Being careful not to take something from a cool spot, spray it, then put it in a warm spot.
4. Sometimes not getting too worried about it - letting the bubbles occur then dealing with them afterwards. This I do by waiting till The affected surface is nearly dry, sanding very lightly to knock the tops off the bubbles, then dragging over it with a pullover mix, then spraying again. The pullover mix drags the wetted lacquer into the craters left by the bubbles. Next coat evens it all up and the pores are mostly filled.

Still, these are a bunch of partial solutions. Like you, I'm still waiting for the surefire one.Arron

Thanks muchly Arron,

The part that puzzles me a little is that this almost never happens on vertical surfaces, only those that are horizontal.

This time I shot myself in the foot. I make a habit of making the first coat of lacquer quite thin, and I could see the pin holes after the first coat. Should have stopped and filled them then, instead of putting on another coat.

Cheerio!

Is there some reason why, on the things you are spraying, the horizontal timbers are more porous then the vertical ones ?

Arron,

I don't think they are more porous ... I think it is me putting on a heavier coat when the surface is horizontal.

Just spoke to a couple of technical blokes from Mirotone .. very helpful. One told me that a leading cause of these bubbles is that the lacquer goes off fast on top, the lacquer under it is still liquid, and the bubbles can force their way to the surface.

They recommended a sanding sealer, lacquer and thinner combo that they reckon will solve most of these problems, so I ordered 20 litres of all three. My current supplier does not have the wide range of lacquer, thinner and sealer types that Mirotone has, so it is worth a shot.

For the record, I ordered 1294 Thinners; 3241 Sealer and 3210 Lacquer; but it is worth noting that this recommendation was for Queensland weather. Other combos might be selected for other areas.

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Originally Posted by John Samuel

Arron,

I don't think they are more porous ... I think it is me putting on a heavier coat when the surface is horizontal.

Just spoke to a couple of technical blokes from Mirotone .. very helpful. One told me that a leading cause of these bubbles is that the lacquer goes off fast on top, the lacquer under it is still liquid, and the bubbles can force their way to the surface.

They recommended a sanding sealer, lacquer and thinner combo that they reckon will solve most of these problems, so I ordered 20 litres of all three. My current supplier does not have the wide range of lacquer, thinner and sealer types that Mirotone has, so it is worth a shot.

For the record, I ordered 1294 Thinners; 3241 Sealer and 3210 Lacquer; but it is worth noting that this recommendation was for Queensland weather. Other combos might be selected for other areas.

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I'd be interested to know what you paid for the product you bought. Pm me if you dont mind telling me but dont want to post it. We can compare notes on what we pay - always better to be well informed when it comes time to negotiate with suppliers.
Cheers
Arron

John, have you had a chance to experiment with your solution. Any improvement ?

cheers
Arron

Arron,

Yes ... much improved ... no more bubbles.

The medium thinners seem to have made a big difference. I was using a fast hydrocarbon based thinner, and it was flashing off on top, but remaining liquid underneath, preventing any bubbles from coming to the surface while the film was still liquid. The slower (non-hydrocarbon) thinners stays liquid longer, allowing the film to "settle in" to the surface and for the bubbles to effervesce out.

But there are downsides. Because the top coat remains liquid for longer, it dissolves more of the existing coat, sometimes resulting in the grain pattern raising a little. Also, I usually use a mix of 70% thinners for the final coat, so it flashes off quickly to a dust free surface. That does not happen with the medium thinners.

So, the solution seems to be to use the medium thinners for the initial coats, and a very fast (non-hydrocarbon) thinner for the final coat.

Another bonus from the non-hydrocarbon thinners is that they are not aromatic, meaning a significant reduction in fumes in the shed when spraying.

All in all, quite happy. I'll get some ultra-fast non-hydrocarbon thinners and all should be well.

Two questions before I can give advice. What is the ambient temp? and are you thinning the top coat?

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Originally Posted by John Samuel

So, the solution seems to be to use the medium thinners for the initial coats, and a very fast (non-hydrocarbon) thinner for the final coat.

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Isnt this solution much the same as using some retarder - only more trouble ?

cheers
Arron

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Originally Posted by kph

Two questions before I can give advice. What is the ambient temp? and are you thinning the top coat?

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About 22 degrees C.

About 15% thinners, as per the specs.

Quote:

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Originally Posted by Arron

Isnt this solution much the same as using some retarder - only more trouble ?

cheers
Arron

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Pretty close, I'd say, but it is no more trouble.

OK,

Using slower thinners helped a bunch, but on one small door I still had a couple of bubbles. I can confirm that this particular problem is air escaping from the timber and forming a bubble under the lacquer. While the lacquer is liquid, these bubbles come to the surface and erupt. But sooner or later the lacquer skins and either a bubble forms under this skin or the lacquer does not flow back after an eruption because the lacquer is too viscous.

The only solution seems to be to properly seal the timber ... but I thought I was doing that by putting the first coat of sealer on thinned out.

So, what other bright ideas are there concerning sealing the timber? Perhaps shellac, but why would that be any better than a thinned out coat of sealer?

John, I'm far from the most experienced lacquer sprayer around but here is what I'd be thinking in your situation.

Basically, you've got two options - deal with the problem before you create the bubbles or deal with the problem after you create the bubbles.

To deal with these last few stubborn bubbles before you create them you would have to seal all your surfaces - completely. You might get away with altering your products till you get a solution that works most of the time, but if you want a methodology that will work every time across all spraying conditions and wont serve you up a nasty, bubbly little surprise when you can least afford it then you have to fill the grain. This is where I find finishers get all mixed up between sealing and grain filling. You need to grain fill really, not seal. You could use a NC sanding sealer like you've been doing but it wont fill all the grain- why would it, its far too thin and I'll bet makes no promises of grain filling. Applying a lot of coats might - but you shouldn't be doing that with a sealer. You could use shellac but its a lousy grain filler - its just a sealer. You could use shellac loaded with pumice or talc or something as a grain filler - but I don't think going down the shellac route is really what you want because it involves multiple coats and is quite labour intensive and I'm guessing you wouldn't be spraying lacquer if you wanted a labour intensive solution. The problem with using this type of thing is that it lays across the hills in the same measure as it fills the valleys, so you need to sand off between coats to get the hills back down to ground zero while the valleys slowly fill. You could use Timbermate, but I don't think it ever looks quite right as a grain sealer, and its grubby, dusty, hard to sand stuff so again its more labour. I'm still looking for the ideal 'sealer with grain filling properties which looks natural and doesn't muddy the look and doesn't involve lots of sanding and frigging around and doesn't go mental when sprayed with lacquer thinner' but I haven't found it after lots of experimentation.

The other option is to deal with the bubbles after they are created. That's what I do when they are limited to a few bubbles in a big job that basically went well. As you can see from my other posts I'm a big fan of the seldom-mentioned potential of pulling lacquer to fix problems. I'd spray, and when the bubbles first become evident, sand lightly over them to break the heads off the bubbles and leave a crater. Then, using my pullover mix and a clean rag, drag backwards and forwards over the general area till the craters are filled with the partially dissolved slurry of lacquer. This might sound like a hassle but it only takes a few seconds and I don't even wait for the coat to dry, better to do it while its still a bit soft. Once I'm done, if the surface is rutted up I'd sand again, but preferably I'd just carry on with the next coat of lacquer and trust that it will redissolve a sufficient amount of the coats below to give me a nice smooth surface. If I don't fill the bubbles first time, I just repeat. If I realise after I've finished spraying that I failed to fill all the craters then I just let it go through to the buffing stage and nail them then. Of course I wouldn't take this approach if I was getting a lot of bubbles, but its what I do when I am getting just a few.

cheers : generally, I hesitate to offer advice as I'm far from an experienced sprayer but that's what I find works.
Arron

Thanks Arron,

I think you are right about grain filling and sanding. My bubbles are definitely coming from "valleys" in the grain ... I have watched them form and erupt. If I fill the grain completely, I don't get bubbles.

What is a pullover mix?

Cheerio!

If you look up or down in this forum you'll see a thread by me on pullover mixes - with a nice definition contributed by another forumite. Basically, its a hand-applied mix that softens and repositions the lacquer after its been sprayed on. My understanding is that it can be applied in one of two ways:
1. by process, across the entire surface. You would do this to get a piano finish (ie dead flat mirror finish). In other words, you are using the softened lacquer to fill the grain. You would do this somewhere before the final coat. Just pull across the entire surface till you have pushed softened lacquer into all the pores.
2. as a spot repair - either during the spraying process or after.

I don't really use method number 1 because I don't want a piano finish. My reasoning is that with most things in the world today looking like plastic I don't want to make my woodwork look like plastic too. Hence I want the grain to telegraph itself through the finish. It probably looks like I would like to get a piano finish but just don't know how to, but I don't mind that. I think a lot of the finishing techniques we hear about originated in the 18th century when there was no plastic to look like, so they didn't have this concern. Having this preference is convenient because whatever process you use to fill grain it is boring and labour intensive.

I do use method number 2.

A couple of other things I have thought about here:
1. lately, I've taken to attaching a second gun while spraying - a touch up gun which I fill with straight lacquer thinner. My idea is to pick this up and spray it across any faults which might appear while I'm spraying, allowing lacquers wonderful tendency to sort out all problems underneath to work for me. I've only used it once when I had a droplet of drool fall my respirator (I must keep my mouth open without realising it) into my work. I removed the liquid, rubbed it flat with my finger and then hit that spot a couple of times with the lacquer thinner before carrying on with a couple more coats. It worked in the sense that it flattened the mess out enough to allow me to fix it during the buffing process. It certainly beats having to sand flat and start again which is the only other solution I can imagine. Anyway, it may be useful if you are repairing bubbles by pulling - give it a localised spray of lacquer thinner, then put it at the bottom of the queue to flash off while you carry on spraying other things.

2. I find nc sanding dust is a good grain filler. When I sand something, say after applying a couple of coats of sealer, I used to clean it down completely before carrying on. Now, I leave the sanding dust in the grain, and just blow the corners clean. The next coat softens the dust and it becomes lacquer again, but deep inside the grain. Obviously you don't want grit or wood dust, but nc dust is a good filler so why waste it.

I should also point out that I have chronic RSI, so I have an aversion to sanding or hard rubbing that probably doesn't make a lot of sense to most people.

cheers
Arron

Managed to patch up two of the problematic panels. The third I sanded back to timber and started again.

Put on a thin coat of sealer, followed by a full coat. Sanded back ... looked OK. Applied another coat of sealer and sanded back again (trying to prevent build-up to too thick a film). By now the grain is all but filled to a smooth flat surface, but I applied another coat of sealer ... and got a couple of bubbles.

Bother, spit and darn!

Sanded the bubbles out and put a small amount of sealer on a can lid and continually worked it with a small brush until it was goopy, then applied this thick material to a small area around the bubbles. That worked. Lacquer went on fine without bubbles. Next time I'll try making up a pull mix.

Sometimes I get pin holes that do not emit bubbles. These are generally easy to fix, I "spot spray" the pinholes to fill them with lacquer and immediately spray the entire panel. Now that I am using slower thinners this seems to work fine for pinholes that are not emitting bubbles.

What I really want to do is to solve the problem of bubbles by effective sealing at the outset. Maybe I'll try two thin coats of sealer at the outset ... maybe not.

I've never had bubbles when spraying with poly or acid cured (high-build) lacquer. Is this what others have found?

Try some sil drops in your lacquer mix you only need two of three drops.You may have a problem with silicone.Worth a try and won't damage your coat of lacquer it will clear the problem up in a flash.

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Originally Posted by foxhound

Try some sil drops in your lacquer mix you only need two of three drops.You may have a problem with silicone.Worth a try and won't damage your coat of lacquer it will clear the problem up in a flash.

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Thanks for that.

However, I doubt the issue is silicon. I have watched the bubbles forming under wet lacquer in the grain, and then rising to the surface and erupting. Still, if the problem persists, I'll try it.

When the bubbles are grain orientated it usually turns out to be silicon.

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Originally Posted by rustynail

When the bubbles are grain orientated it usually turns out to be silicon.

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Thanks Rustynail and Foxhound,

I was not aware of that. I have been careful to try to keep silicone out of my shop with one exception, the silicone sealant used to seal my dust collection system. Lubricants, polishes etc containing silicone are banned from the shop.

Did a search for sil drops. Only found a reference to Silastic Replicating Resin (SIL-FLO) and brazing products. Can anyone help me understand what sil drops are and where I can get them?

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Originally Posted by rustynail

When the bubbles are grain orientated it usually turns out to be silicon.

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I would have thought that if the issue was silicon contamination John would be getting fish-eye rather then bubbles (fish-eyes = round bare patches where the finish fails to adhere). How does silicon contamination lead to tiny enclosed bubbles arising deep within the pores?

Cheers
Arron

Hi John,

Sorry for the late reply but you should be able to get the sil (silicon) drops from a panel beaters shop or the likes.The last time I purchased a bottle it was from Wattyl.

Thanks Foxhound,

I'll try it and see what happens.