how to apply polyurethane with a spray gun
- Thread starter ukjo90
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AES
Established Member
Do you mean a clear poly varnish, or a paint? I will be spraying poly colour soon now (2 part) and though I have a fair bit of experience I would hesitate long and hard before saying it's faster! Add in the prep (masking, etc) and cleaning the gear afterwards, and IMO it's anything but (BTW, I'm not talking HVLP here, but a compressor driven touch up gun by SATA, plus a Badger airbrush for some small areas).
You will also need some supplementary gear (ideally a viscosity cup), strainers (old tights), mixing cup/s, AND a method of accurately measuring the original mix, PLUS the amount of thinner you end up requiring to spray successfully - trial and error, along with nozzle size (if applicable) paint opening (ditto) and pressure.
Also, unless you've sprayed before it's a mistake to think that "just" using a spray gun automatically results in a good finish. It doesn't - at least not without a fair bit of practice first!
Note please that I'm talking about colour here - clear varnish MAY be a bit easier, certainly re masking, and PERHAPS re mixture prep.
I suggest you have a look around on the Internet before going further. There's lots of good stuff about spraying there, including by our own Peter Millard and his excellent "10 minute workshop" series. But note he uses water based paints (AFAIK polyurethane is not available water based) and note also the spray gear he uses- MUCH different to my own.
Finally do NOT forget that even if not actively harmful to your health, you definitely do NOT want to be breathing in spray fumes all day, especially NOT 2-part poly, and neither do you want to be (trying to!!!) clear up overspray. Outside spraying is good in some respects, but this in NOT now the weather for that (temperature and humidity) and I really would think twice about spraying any sort of poly indoors unless the job is very small and you have very good extraction, etc.
Sorry to rain on your parade, I am a fan of spraying - sometimes - but if you're a newby to it there's MUCH more involved than you probably realise. With care a GOOD brush job, or even a roller can produce end results almost as good (IMO).
HTH, good luck.
You will also need some supplementary gear (ideally a viscosity cup), strainers (old tights), mixing cup/s, AND a method of accurately measuring the original mix, PLUS the amount of thinner you end up requiring to spray successfully - trial and error, along with nozzle size (if applicable) paint opening (ditto) and pressure.
Also, unless you've sprayed before it's a mistake to think that "just" using a spray gun automatically results in a good finish. It doesn't - at least not without a fair bit of practice first!
Note please that I'm talking about colour here - clear varnish MAY be a bit easier, certainly re masking, and PERHAPS re mixture prep.
I suggest you have a look around on the Internet before going further. There's lots of good stuff about spraying there, including by our own Peter Millard and his excellent "10 minute workshop" series. But note he uses water based paints (AFAIK polyurethane is not available water based) and note also the spray gear he uses- MUCH different to my own.
Finally do NOT forget that even if not actively harmful to your health, you definitely do NOT want to be breathing in spray fumes all day, especially NOT 2-part poly, and neither do you want to be (trying to!!!) clear up overspray. Outside spraying is good in some respects, but this in NOT now the weather for that (temperature and humidity) and I really would think twice about spraying any sort of poly indoors unless the job is very small and you have very good extraction, etc.
Sorry to rain on your parade, I am a fan of spraying - sometimes - but if you're a newby to it there's MUCH more involved than you probably realise. With care a GOOD brush job, or even a roller can produce end results almost as good (IMO).
HTH, good luck.
Sgian Dubh
Established Member
Oil or solvent based polyurethane varnish can certainly be sprayed, assuming that's your plan. It's something I've done quite a number of times over the years. It's generally too viscous straight out of the tin so needs thinning, for which I've normally used white spirits. As to how thin, my target is thin enough that the varnish runs off a stirring stick pretty rapidly in a steady unbroken stream until it starts to break up for the last three to five seconds.ukjo90":1m9we6ge said:
In general, my strategy has always been to reduce both air pressure and fluid delivery to the tip somewhat for spraying varnish in comparison to the settings I use for pre-cat and AC type lacquers. In other words reducing these settings lets you gently apply the finish and similarly gently build up its thickness. It's a fine line between getting the varnish on just the right amount and too heavy: this is real challenge on vertical surfaces because the finish is so slow drying that if you apply it too thickly you'll end up with major drips, sags and curtains. However, all is not lost if you get this through poor technique because if you get plenty of clean white spirits, a rag, and protective gloves you just wash it all off and let the surface dry and have another go, perhaps after a bit of minor repair through light sanding.
Generally, my advice is start applying the finish at the top surfaces and work down to the bottom, but don't omit to cross-hatch as normal, and be aware that spraying varnish results in lots of very sticky overspray that settles on all surfaces nearby and extraction really is a must as you spray to suck the overspray away so that as little as possible settles on nearby horizontal surfaces.
Finally, I really don't like spraying varnish because of the challenges I've described above when working with such a slow drying finish, but if it's done right you can end up with a beautifully applied finish, but there's still the chance of it ending up less than ideal through the usual risk of dust, bugs, and other sundry contaminants landing in the wet finish, which usually takes three or four hours to cure to barely touch dry. Slainte.
As a rule you can spray anything, even if the original manufacturer says you can't or shouldn't :lol:
But as you specifically mention your goals are greater speed and a nicer finish have you heard of wiping varnish? If not you might want to look into that as an alternative.
Converting oil-based poly to wiping varnish by diluting it a bit makes it faster and easier to apply, and then the heart of the technique – wiping away some or all excess – virtually guarantees good results. Very even application is assured, including on inside corners and vertical surfaces with no risk of drips or runs, plus it minimises problems with dust nibs during drying.
Usual starting dilution is 1/3 to a 1/2 (1/3 is said to equal the typical commercial versions) although you can dilute as much as you like if you prefer to work with it very thin.
Its only real downside is the number of layers required if you want to build a full varnish coat, because each application is so thin it takes many to build up an equivalent to 3-4 coats applied straight from the tin by brush.
But as you specifically mention your goals are greater speed and a nicer finish have you heard of wiping varnish? If not you might want to look into that as an alternative.
Converting oil-based poly to wiping varnish by diluting it a bit makes it faster and easier to apply, and then the heart of the technique – wiping away some or all excess – virtually guarantees good results. Very even application is assured, including on inside corners and vertical surfaces with no risk of drips or runs, plus it minimises problems with dust nibs during drying.
Usual starting dilution is 1/3 to a 1/2 (1/3 is said to equal the typical commercial versions) although you can dilute as much as you like if you prefer to work with it very thin.
Its only real downside is the number of layers required if you want to build a full varnish coat, because each application is so thin it takes many to build up an equivalent to 3-4 coats applied straight from the tin by brush.
Keith 66
Established Member
If you are going to spray 2 pack poly you need to look at the health implications, the isocyanate hardener is lethal. An air fed respirator is recomended, carbon filter ones are ok for small jobs but dont last long, basically as soon as you can smell the stuff you are getting too much.
I sometimes used to use it for spraying boats but never liked using it, too many horror stories. One bloke i know personally now has chronic COPD & asthma due to exposure to 2 pack with no mask & to hear him fighting for every single breath every minute of every day is the best h&s lesson anyone should need. I will not spray the stuff now.
I sometimes used to use it for spraying boats but never liked using it, too many horror stories. One bloke i know personally now has chronic COPD & asthma due to exposure to 2 pack with no mask & to hear him fighting for every single breath every minute of every day is the best h&s lesson anyone should need. I will not spray the stuff now.
