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Thread: Question about spraying Polyurethane with HVLP

  1. #1

    Question about spraying Polyurethane with HVLP

    I'm pretty new to the whole HVLP spray finish scene but I picked up a used Sprayfine fairly cheap. I've used it to spray some typical Minwax Polyurethane and got good results by thinning it slightly but it takes so long to dry. Is there a trick to getting it to cure faster? More thinner or should I try a different product?

  2. #2
    Join Date
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    Standard varnish, which is what you're trying to spray, is designed to have a long cure time. That's so it stays fluid while you're brushing it on. For spraying, typical finishes cure much more quickly. I mostly use General Finishes Enduro Clear Poly. A coat of it will cure to the touch in ten minutes or so, and cure enough to be sanded in an hour or two, depending on temperature and atmospheric humidity.

  3. #3
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    Quote Originally Posted by Dan Jansen View Post
    I'm pretty new to the whole HVLP spray finish scene but I picked up a used Sprayfine fairly cheap. I've used it to spray some typical Minwax Polyurethane and got good results by thinning it slightly but it takes so long to dry. Is there a trick to getting it to cure faster? More thinner or should I try a different product?
    Varnishes "dry" in two phases: First the volatile components (including any thinner) evaporate leaving the resin/oil solids behind, and then the solids cross-link (chemically bond) to form a solid film. The second part is what takes so long, and mucking with the volatiles by adding thinner etc won't do a thing to change it.

    What you want here is finish that quickly cures to a point where it can be handled/recoated/sanded and doesn't attract dust. There are two basic ways to do that:

    1. If you want to stick with oil-based varnishes, then step up to a catalyzed finish of some sort. The catalyst greatly accelerates the cross-linking process.
    2. Change to a finish that cures by a different mechanism, for example one that cures by evaporation (lacquer, shellac) or by coalescing (many non-catalyzed waterbornes).

  4. #4
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    I've sprayed water based poly with good success. One complaint about WB poly is that it doesn't impart the amber tone of oil based poly. I tried a WB poly from Home Depot that was supposed to produce the amber tone of oil based poly. Eh, not really. As luck would have it, transtint works just fine in WB poly to yield any tone you like, it just takes some experimenting.

  5. #5
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    Spraying an oil based varnish can be a thankless task because of the slow dry times that have already been mentioned..."sticky city" wherever any overspray lands. The long dry time also limits the number of coats one can apply within a given time period. It's not "hard" to spray, but the after-effects are what they are. Water borne finishes are ideally sprayed by contrast. They lay down nicely with a reasonable quality gun and reasonable skill and dry to the touch relatively quickly. That means much of the overspray is dry or nearly dry before it hits the 'floor" (or whatever) and you can re-coat quickly to build your finish to where you want it to be. Shellac and solvent based lacquers are similarly easy to spray (and desirable to spray) although the latter requires levels of safety consideration that most home shops do not provide.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  6. #6
    I've had good success emulating the warmth of oil-based finishes by spraying a seal coat of blonde shellac under Enduro Clear Poly. It's quite passable.

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