Bubbles in NC lacquer

[John Samuel]

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?

[Arron]

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

[John Samuel]

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!

[Arron]

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

[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.

[Arron]

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.

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

[Arron]

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

cheers
Arron

[John Samuel]

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.

[kph]

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

[Arron]

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.

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

cheers
Arron

[John Samuel]

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

About 22 degrees C.

About 15% thinners, as per the specs.

[John Samuel]

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

cheers
Arron

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

[John Samuel]

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?

[Arron]

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

[John Samuel]

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!