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Strange behavior in finish when HVLP spraying

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Forum topic by jonlan posted 12-08-2019 02:41 PM 472 views 1 time favorited 19 replies Add to Favorites Watch
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jonlan

72 posts in 1494 days


12-08-2019 02:41 PM

Hi folks. I’m making some built ins for around our fire place and my wife wanted them to match the current molding in the house which is Sherwin Williams Pro classic oil based paint. I don’t love working with the stuff but I’ve found that I can spray it with my HVLP when thinned a little with paint thinner and it seems to go down well. I started with a base coat of Sherwin Williams primer. Sanded it down to 220 after dry, and then put down my first coat of the oil based paint. I’ve noticed this strange sort of issue in the finish in a couple of places and I can’t sort out what I did wrong. Initially I thought that I had laid down too much paint and it was sort of bunching up – but I then saw it happen in another area where I had applied a very light coat.

Does anyone have any idea what might cause this? Perhaps an insufficient amount of primer? I did sand through the primer in a couple of places trying to get the edges to look right but I assumed that since I had already raised the grain and sanded it with the primer that I’d be OK. I should note that in 95% of the other places the finish went down great and looks awesome.

Im at a loss as to what I did wrong or what might cause this. Any ideas would be appreciated.


19 replies so far

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splintergroup

3207 posts in 1829 days


#1 posted 12-08-2019 03:35 PM

Those look like tiny popped bubbles.

My only guess is the area has the primer thinned out too much with sanding. The bubbles form when air trapped in the wood pores seeps out (breathes), possibly from some temperature differential between the project and the sprayed material?

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jonlan

72 posts in 1494 days


#2 posted 12-08-2019 03:39 PM

Thanks for the reply @splintergroup

Interesting. So perhaps my mistake here was to sand through the primer. I guess I assumed that even though I did in some places that the primer had already done its job and would have filled the pores.

I’ll also note that I live In MN and I had a vent fan running blowing air out of the space I was spraying in so there was some cold air coming in at the same time. I guess I’ll try making sure I dont sand through the primer next time.

In terms of fixing this – I guess I’ll wait for it to dry, sand it down and hit it again.

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LeeRoyMan

498 posts in 334 days


#3 posted 12-08-2019 04:18 PM

To me it looks like rough wood that wasn’t sanded completely smooth to start with, but I didn’t see it before so it’s hard to tell.
Also looks like you’re spraying pretty thick.

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jonlan

72 posts in 1494 days


#4 posted 12-08-2019 04:20 PM

Yeah – admittedly Im guessing in this spot that I likely saw the initial issue and thought the issue wasnt enough paint so Im guessing I hit it again and got too much on there.

Im wondering if I should have done more surface prep. I didnt use a tack cloth before spraying on the wood – but I did hit it with the air hose to get all the dust off. Would it be a good idea to use a tack cloth? Or should I just wip e the surface down with mineral spirits?

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LeeRoyMan

498 posts in 334 days


#5 posted 12-08-2019 04:29 PM

Just out of curiosity, Is the wood pine/fir?

I don’t like tack rags myself. I would just blow it off then use a very lightly (MS) damped cloth to wipe them down, then another light blow to get any cling-ons off from the cloth.

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jonlan

72 posts in 1494 days


#6 posted 12-08-2019 04:31 PM

It’s poplar trim and the center is maple faced plywood. I think I’ll give the mineral spirits a try and see how that works. I also just realized I forgot to put my inline water separator inline behind my gun when I sprayed. Although if that was the issue you’d think I would have seen it all over.

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John_

225 posts in 2313 days


#7 posted 12-08-2019 06:25 PM

Looks like your spraying too heavy of a coat or the paint isn’t atomizing properly.

Have you ever tried something thinner like Benjamin Moore Advance?

What are you actually using to spray?

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CaptainKlutz

2247 posts in 2101 days


#8 posted 12-09-2019 03:01 AM

+1 see tiny popped bubbles.
Also see orange peel in other areas.

Orange peel AND bubbles are often related to moisture intrusion while spraying an oil base.

Orange peel alone is usually due less than optimal atomization, where solvent(s) are evaporating before paint could level out. Common solution (besides better spraying) is to add a reducer that is blend of solvents that evaporate at different rates to aid flow out based on spraying conditions. Not using the proper reducing solvent for spraying only makes the job harder. Instead of paint thinner, SW sells several reducers for oil base paints.

One puzzle: The bubble defects on top piece appear in straight lines that would aligned with sap/heart wood boundaries in underlying wood. Which could also mean that the wood grain had water/solvent in pores, that was released while spraying. No way to know if due to over sanding primer or not? Could be related to pores in paint holding more material due too heavy of coating and more solvent evaporating after paint had flowed out.

One spraying tip: you want a uniform primer surface under paint. Auto body work always sprays one final primer/ sealer coat – with zero sanding, before the put down base coat. The sealer lets you cover any variation in surface absorption created by sanding.

Best Luck.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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jonlan

72 posts in 1494 days


#9 posted 12-09-2019 03:02 AM

Im using a Ingersoll Rand HVLP driven off my compressor. I’ve never had great luck at running any HVLP gun at low pressure. Always seems to get a smooth finish that I need to get the pressure up toward 30 or 40 PSI which always seems too high to me but I’ve played around with it so much and that’s the only thing that gave me what I wanted.

I think I must have had some surface issues with the first spray. I sanded tonight and laid down another coat but did so with many lighter coats. Looks OK now – I guess I’ll see in the AM!

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jonlan

72 posts in 1494 days


#10 posted 12-09-2019 03:14 AM

@CaptainKultz . Thanks for the feedback! I’ll admit that as much as I try to “take it slow” i often end up rushing things. My view of primer has always been to raise the grain and help fill in voids to give an even surface. With that mindset – my thinking was that after the primer dried it had done its job and I could sand it down as needed. My thinking was that in the worst case scenario I’d had to put on another coat of top coat to cover up any areas that I needed to (where I had sanded through the primer). What you say makes good sense to me though.

The issue I have is that I have had a hell of time HVLP spraying SW primer. I can’t recall if I had previously tried thinning it (although I’m sure I have) but I even went as far as buying a dedicated HVLP gun with a larger jet (needle set?) to get it to pass through the gun correctly. In any case – it does the job of getting the primer on the project, but I don’t think it’s very pretty. But again – I didnt care since I knew I’d be sanding it. Perhaps I should talk to them about thinners for both the primer and the oil based paint and try that? I think in the past when I’ve gotten the primer to go on it wasnt smooth either hence more sanding. Maybe I do just need to thin it down. I agree that having a final “primer” coat would likely help with most of my issues.

The other problem I have is the actual application of the paint. Once I get passed the first coat, I have a heck of a time seeing the paint lay down. I’ve tried using dedicated work lights at different angles but I can never quite seem to see where I’ve been. This combined with my poor spray technique (which Im working on) is definitely hurting me. I’m trying harder to be more consistent and do full passes in one spray each time but again with not being able to see the coat going on this is challenging. I spend a lot of my time wondering if the gun is even shooting and feel the need to check often to make sure it is.

I’d be open to any tips you have on that matter as well.

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CaptainKlutz

2247 posts in 2101 days


#11 posted 12-09-2019 04:41 AM

1st glance, your problems read like your gun tip is too small, and you need practice.

Not seeing the paint on surface is also worrisome. Guessing you need to to add more side lighting to your paint booth. With proper light angles, even if you can’t see the clear coating, you can a least see the wet sheen as it deposits and trail it leaves behind.

BTW #IAMAKLUTZ, not a spraying expert, but will offer a couple things I have learned.

1) Industrial paint suppliers will almost always have recommendations for tip size, gun type, and even pressure. Paint gun mfg also have recommendations. Asking your local supplier is usually good too, as they will know how to adjust based on local weather conditions.

That said, primers have fillers in them to smooth out surfaces. They need a larger tip size than a clear finish. While you can push primer through a 1.5mm tip, a 1.8-2.2mm is much better. But if you attempt to push thin lacquer through same tip, you get orange peel. For thinner paints like a lacquer, 1.4-1.6mm is more typical. What size to use also depends on type of gun. Airless systems can use smaller tips, than the gravity fed HVLP suggest above.

2) Thicker, heavy body paints like alkyd enamels require much higher level of atomization than thinner auto body enamel, clear, or lacquer. Orange peel is primary defect when poor atomization or too much material per pass happens. Have seen horrible orange peel issues in AZ due low humidity, spraying outdoors. Both my industrial paint suppliers suggest using airless, or turbine HVLP gun for alkyd enamels or WB poly as solution to low humidity orange peel. They produce more volume than a compressor based gun, which usually means better atomization. Based on this advice, picked up a used pressure pot feed Binks HVLP, and I have needed less reducer to get same results as my gravity fed HVLP guns. Alkyd paint is still temperamental, but it’s better.

3) If you can’t get better atomization, you need to properly thin the finish material. While you can use many reducer solvents to thin paint, they have to picked based on temp/humidity conditions at time of use. I have an entire shelf of various solvents, and proprietary reducers I use depending on when and what is being painted. Being a bit of chemistry geek, even have a chart of solvent evaporation rates and boiling temp properties that I use to help me pick the right solvents, and/or understand why various reducers have a solvent blend that works in different conditions, while others fail.
https://producerschemical.com/media/PC%20Solvent%20Chart.pdf
In cool weather, I want fast solvents. In warm weather, want slow solvents. Even with my past learning, In hot weather with temp is over 90°F, It’s almost impossible to spray heavy body finishes without some orange peel. So I don’t even attempt spraying when conditions are not right.

Hope this helps a little. If you want get best advice, talk to people who make/sell your paint/finish materials. They have a vested interest in your success. :-)

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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jonlan

72 posts in 1494 days


#12 posted 12-09-2019 04:53 AM

Great info. Thanks for sharing all of this!

Each time I talk to the folks at the local SW store they suggest I look at airless systems. Not the first time I’ve heard the recommendation but Im pretty set on not switching systems at this point. I ought to be able to get this to work – but it’s clear to me that I ought to get better about thinning and get more practice to improve my technique.

The one thing Im wondering about now though is how much paint to put down in one pass. It’s clear to me that thinner passes are the way to do it. But after doing the entire piece in one light pass, do you do another right away? Is it like applying urethane where you want a “wet” look? I was just doing some reading and they were suggesting doing multiple passed over the entire piece to get a coat that will settle – but to do so in many smaller passes. AKA – thin pass over the entire piece. Then start again, and again – as many times as you need to do in order to get a look that looks like it will settle. If thats the case – then I need to figure out how much one needs for it to level :)

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CaptainKlutz

2247 posts in 2101 days


#13 posted 12-09-2019 05:31 AM

The gold standard in setting up your spray process thickness is wet film thickness gauge. Can find them at any decent paint supplier. There are many types to choose from, even disposable ones.
https://www.gardco.com/filmthickness.cfm#wetfilm

The calling card style is popular. My SW and PPG stores will offer me one free occasionally when buying a larger order, if they don’t try asking. It’s essentially their business card with address and phone printed on one side.

Different paints have different optimum wet film thickness. depends on polymer, solvents, fillers, pigments, etc; in the formulation. Generally, when you go over 3-5mils thick with clear wood finishes, it tends to run. Auto base coats usually tolerate less thickness, primers a little thicker; before then run/sag.

Optimum film build is usually part of the use data available from finish supplier.

Cheers!

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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John_

225 posts in 2313 days


#14 posted 12-09-2019 07:38 AM

HVLP spray gun is usually set to 30 psi. You really need to test spray a cardboard box or some construction paper, for example, to see how the pattern is looking. Here are some examples:

I always thought that the whole idea of using oil based paint is it takes longer to dry which allows it to level out (aboid brush marks, etc) It is clearly not happening.

I would strongly suggest you switch to a different product. Again, Benjamin Moore advance is a lot thinner and easier to spray, but their is a disadvantage in that it has a 24 hour recoat time and may take several weeks to fully cure. Another option that I have had excellent results is General Finishes makes a ‘pigmented’ poly. It comes in either black or white in 3 different sheens (satin, semi and gloss) It really is easy to work with, sands fairly easy and can be recoated in a couple of hours.

Lastly, my big ‘breakthrough’ in spraying wood finishes came when I finally moved up to a pressure pot type of gun. Being able to force the fluid to the gun made all the difference in the world. I have actually heard good things about the Harbor Freight guns – here is an example (I have a C.A. Technologies gun and pot that I have been very pleased with)

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Robert

3602 posts in 2087 days


#15 posted 12-09-2019 02:28 PM

What size is the tip? For cabs you should be using a small tip like 1.4mm.

I’m an admitted rookie, but I have sprayed the ProClassic oil based successfully but I found the key is to thin it.

I’m now beginning to realize just how thin the paint has to be – 30-40 seconds through a Ford cup. If you’re having to jack up the pressure this could be the reason.

BTW I’ve pretty much gone to water based Pro Classic Alkyd or Target Coatings pigmented wb lacquer. Of the two the TC is much easier to spray.

Of course wb paint has its own issues…...

-- Everything is a prototype thats why its one of a kind!!

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