HVLP question spraying water-based poly

[blueman]

As a newbie to HVLP, I have encountered the following problem... I am spraying Minwax (water-based) Polycrylic semi-gloss polyurethane.

Although the spray seems to atomize well, it seems that unless I put on a fairly heavy coat that it deposits in discrete though very small droplets resulting in a somewhat rough surface. i.e. - the spray doesn't seem to coalesce into a uniform coat unless I spray it on pretty thick. It's almost if some type of surface tension is causing the spray to coalesce together rather than completely and uniformly wetting the surface.

When I move the gun slower to let more finish deposit, I do get a uniform sheen that completely 'wets' the surface (however this only works when the surface is horizontal because otherwise it would run).

- Is this normal or am I doing something wrong?

- If so what are the likely causes and corrections?

Note: I am using the low cost but surprisingly well reviewed Harbor Freight #66222 (purple) gun. My 8 gallon compressor seems to have enough reserve to power the gun since it is able to cycle. on and off and the pressure never drops below about 70psi.

The surface I am spraying is clean, well-sanded Birch plywood.

I have the inlet pressure set at about 30PSI.

I have made some (non-exhaustive) attempts to:

- Adjust the inlet pressure in the range of 20-40PSI

- Adjust the fluid volume

- Adjust the spray pattern (more round vs. more oval)

- Adjust the distance to the object (from maybe 8 to 16 inches)

- Adjust the speed at which I sweep across the object

While Minwax claims it hasn't tested and hence can't recommend polycrylic for spraying, others have had good results with it. Also, it seems to be of fairly high viscosity and hence seems to spray well.

Thanks

[Robatoy]

I have sprayed the same product with a 2.0 tip on an HVLP gravity feed gun at 45 psi inlet pressure. Medium fan. It works a lot better in a cooler environment.

[nailshooter41]

To get a good, comprehensive answers to your questions, it would be easier for you to simply search this group. The subject of HVLP spraying of water borne materials has been discussed completely several times.

Any time you have spraying errors you can have a *multitude* of problems that will generate poor results. Some mimic others, so it is best to get a list of common spraying problems and their diagnosis and begin from there with your own equipment. I spent a lot of time writing out the way to properly set up your gun for spraying a long while back, and I was contacted by more than a few that thought the info I put out was helpful. It might be to you as well.

FWIW, I think Rob may be on to something. MOST guns (not all) that come with one tip use a 1.4 to 1.7 mm tip. If you are in that range, it might not be large enough to spray your finish without thinning. If you didn't get another tip with the gun (the 2mm is intended for unthinned latex), try thinning your material about 10% with distilled water, then shooting it.

I would also suggest that you find a good book(s) on spray technique. There are a lot of good ones out there, and they can help you troubleshoot your problems. Some of those books will help you realize that there are too many variables to troubleshoot ACCURATELY without a ton of info. They may also be able to help you figure out what you are doing wrong as well.

Plan on investing some time and money in learning the spray process. It is no different from any other part of learning a craft, but it seems to be the biggest enemy of wood workers as they feel like it is something that they can learn from osmosis. You can spray material out of a gun by simply hooking it up. But to shoot a well laid, consistent finish is something that takes time, practice, and above all, patience.

Robert

[J. Clarke]

Note that getting larger tips for the HF gun may be problematical--I tried to get a larger one for mine and the gun had been discontinued as had the tips. They may be available for the replacement model.

While the HF purple gun is fine _if_ out of the box it does what you need, I've found that tcpglobal has what appears to be the same gun for not a lot more money and they have a wide range of tips available, as well as repair kits and accessories. Their tips work on _my_ HF purple gun but mine isn't the current model.

Note--assume that the regulator that comes with the gun is dead, not just with TCPglobal but HF as well--I've gotten one good one out of four guns purchased.

If the tip is too small then you pretty much have to thin.

[Larry Jaques]

Condolences. Perhaps you could thin it with Floetrol. I believe that Naily has sprayed that and will be piping up soon.

-- Experience is a good teacher, but she send in terrific bills. -- Minna Thomas Antrim

[Robatoy]

I bought that 2.0 gravity gun specifically for squirting Sikkens 3+1 primer. An automotive product that just works wonders on MDF's edges and routed profiles. The build is insane, the sandability fantastic. (For some reason my spellchecker balks at sandability...oh well.) That stuff is so thick, it needs a tip that large. It's an Iwata LPH440-201. I took it, all nice and clean to my buddy's booth as I needed to blow some thanes on a few sheets of luan. I checked the Polycryl with a viscosity cup and looked almost too thin for the 2.0 (IMHO, a 1.8 would have done it too). I wasn't about to dirty up his pot-fed equipment. (Waterbased snot in HIS guns? He would have murderized me.) It went on so nice even though I was using a LOT of air to blow that snot apart.

I have revisited the same spray schedule since with the same results.... and if it works, don't mess with it.

[dadiOH]

FYI, polycrylic is not polyurethane. It is acrylic and urethane is *MUCH* harder and therefore scrarch resistant.

[J. Clarke]

Don't assume that harder means more scratch resistant. And polyurethane coatings come in a wide range of hardnesses.

[cm]

I have had better luck spraying Fuhr water based products. I sprayed polycrylic and it never leveled out as nicely as Fuhr.

Cm

[Max]

Especially if you're after that fuhrry look. :-)

Max

[Robatoy]

Ahw, fuhr crying out loud!!

[Larry Jaques]

Poly-shades/crylic are a fuhr cry from decent finishes.

-- Education should provide the tools for a widening and deepening of life, for increased appreciation of all one sees or experiences. It should equip a person to live life well, to understand what is happening around him, for to live life well one must live life with awareness. -- Louis L'Amour

[Robatoy]

=A0 =A0 =A0 =A0 =A0-- Louis L'Amour

Those finishes are targeted at lazy people who want quick 'n dirty. They excel at that job. French polish? Nope.

[dadiOH]

Scratch resistance is one of the defining characteristics of hardness.

[J. Clarke]

So tell us the engineering definition of "hardness" and quote your source.

[krw]

"The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science."

[Robatoy]

Not to get all 'clarke' about this, but tire rubber is, in terms of hardness. very soft but very scratch resistant.

[-MIKE-]

You succeeded, though. :-)

"one of" not "the"

[Robatoy]

HEY!........ouch!

[-MIKE-]

Happy Thanksgiving! :-p