HVLP - Polyurethane woes


I am using my HVLP system for the first time, and am trying to shoot a book shelf with Varathane oil based gloss polyurethane.
Uuuugh.
First few coats went on pretty good. The third, which I hoped would be final, was pretty good, but not perfect so I figured... wet sand and try once more. Well... since then I have had nothing but trouble. That was 3 coats / attempts ago.
I *think* my problem is that I have thinned it too much. I shoot onto a test board, and I see the finish go from real speckly / dusty looking, to bigger "splotches", and then too a fairly uniform "wet" look... but even if I stop applying the finish at that moment, the finish runs.
So... it seems like when I have applied enough finish to get it to "flow" together to make a nice smooth surface, it is too wet, and I get runs.
Any experienced shooters have any thoughts?
I thinned with Mineral Spirits. Should I be thinning with something else?
Any help or comments (or even words of encouragement :-) would be appreciated.
Thanks, Buzz
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Buzz Burrowes wrote:

sounds like you are applying way too heavy a coat. what percentage are you thinning the poly to?
Dave
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But... if I apply any less my coat feels like sandpaper. When it dries it almost feels dusty... if that makes any sense.

The docs I got with the HVLP system said to look for 18 to 24 secs with a #2 zahn (included with the system).
I think I am shooting stuff that is thinned down to 18. Earlier I was shooting it up closer to 22. On one of my first few coats I got a sag, so I thought maybe the finish was too thick.
Thanks, Buzz
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I understand your frustration. When I first started using my FUJI I had much the same problems. There is a funnel-like device that came with mine that is used to determine the appropriate viscosity of the finish with each nozzle/needle size. I used that device first off but found, eventually, I could thin just 'eye-balling' it. I make it a practice of going over the piece with just a light 'dusting' coat and letting it set for about 10 minutes then I apply a thicker coat. It is best to apply several THIN coats than one thick one... After the thicker coat has well set (6-8 hours usually for me), I rub it down with one of those maroon 3-M sanding pads and wipe the dust off with a tack cloth. I then apply the next thin coat and repeat the process. If I get a run, which I generally do, I try to wipe it off a little while it is wet.. then the next coat usually covers it. Remember if you get any bad runs you can wipe it off with a cloth damp with mineral spirits and start over...just don't wait until the run is hard and try to sand it off.
...good luck...with practice you'll get good results...
...just my 2 cents...YMMV...
larry
--
Columbia, MO
www.llhote.com
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Lawrence L'Hote wrote:

Thanks. I'll give this technique a try.
Buzz
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Hi Buzz, It sounds like a problem I had when I started. The finish was not being atomized small enough. My problem was solved by increasing the air flow and changing to a different needle/ nozzle and air cap. This creates a wider and thinner coat from the same amount of finish. For future projects you may want to consider a WB product as they dry in about 20 minutes giving less time for the dust to settle into the finish. Cheers, JG
Buzz Burrowes wrote:

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On Sat, 30 Jul 2005 12:32:34 -0700, Buzz Burrowes

It is beyond the abilities of most HVLP units to spray poly within the specifications described by the coatings manufacturer.
Essentially, the specs that you are spraying to come from the manufacturer of the spray unit.
Poly usually does not allow the degree of thinning needed by the HVLP rig. To thin according to its needs will destroy the chemistry of the coating.
I have a three stage unit, with a decent assortment of caps, and none of the combinations that I have tried have given satisfactory results.
I have had better results with a high pressure gun/compressor setup.
Tom Watson - WoodDorker tjwatson1ATcomcastDOTnet (email) http://home.comcast.net/~tjwatson1/ (website)
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That's why wipe on poly doesn't work. Right?
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I never used wipe on poly.
Are you supposed to thin it?
Tom Watson - WoodDorker tjwatson1ATcomcastDOTnet (email) http://home.comcast.net/~tjwatson1/ (website)
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On Sun, 31 Jul 2005 16:45:53 -0400, the opaque Tom Watson

True. If used, poly should be sponged on.
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Tom Watson wrote:

A high priced sprayer such as an AccuSpray will spray poly with fine results. all HVLP's are NOT created equal. Granted it's expensive, but hey, it's only money. :)
Dave
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Try thinning with a small amount of naptha instead of mineral spirits, or try Penetrol a viscosity additive available at most paint stores. Also keep the spray moving to prevent too much build-up in one spot. Dave.

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On Sun, 31 Jul 2005 18:42:05 -0400, "dave lindsay"

I've used Penetrol but never heard of it being used with poly.
Naptha will dry faster than mineral spirits, which may be a problem in this use.
Whenever I have a finishing problem I go to Jeff Jewitt's website, or call him up - he's da man.
Tom Watson - WoodDorker tjwatson1ATcomcastDOTnet (email) http://home.comcast.net/~tjwatson1/ (website)
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