question on spray nozzle for catalyzed urethane - created 04-08-2008

Hoeven, Eric van der - 04/08/2008.15:54:01
overworked and beyond repair

I've worked extensively with traditional finishes, but want to start using

new spray equipment and a 2-pack Urethane lacquer.

The particular finish I'm refering to is CLOU cloucryl high brilliance 17465.

(I can't get McFadden L-4108 on this side of the atlantic pond).

The CLOU website recommends using a 1.8-2.0 spray nozzle with this finish.

I am told by a painter that this is way too big for spraying guitars.

I am wondering how important is nozzle size on HVLP? Can I use a smaller nozzle and up the pressure? Please help, as I have no experience spraying catalyzed lacquer, and a new spraygun is a major investment.

I have no experience with that finish, but would like to make one point. Take recommendations from finish suppliers and finishing equipment manufacturers with a large grain of salt. They are used to the large surfaces and continuous use demands of professional furniture and case work finishers. Guitars finishing is VERY different in more than one way.

That said, a smaller nozzle (and needle, they come as a set) sounds reasonable. You can always thin the finish if you're not getting enough material in the spray pattern.

What Barry said.

Just for discussion sake:

>I am wondering how important is nozzle size on HVLP?

Very important, and not only for HVLP. Tip (and needle) size will need to be adjusted to the material that you're spraying, pressure, gun type, . . .

>Can I use a smaller nozzle and up the pressure?

Just because you use a smaller tip, doesn't mean you need to up the pressure. Depending upon what kind of HVLP (especially conversion), the internals of the gun is designed to spray not only at a certain pressure, but also a certain volume. A smaller tip size will limit your amount of material flow - a good gun will allow adjustment for not only proper air flow and spray pattern for the amount of material being sprayed, but also how the gun atomizes the material. While that last adjustment may take you a long time to get right, once its done, you'll be glad you spent the time.

I can tell you this, from a language standpoint, the Clou site confuses me. Urethane and lacquer are two different materials, so are you spraying urethane, or are you spraying lacquer? In automotive, urethanes use reducers (and usually hardeners), and lacquers use thinners.

And, ditto to what Mario stated.

What Aaron said. (I think I closed the loop)

I shoot Dupont catalyzed polyurethane using a hvlp gravity feed "door jam" gun that has a .8 mm tip. It seems to work just fine.

Most finish companies have servise reps and open classes. They want you ot buy their stuff and shoot it thru their guns, so there is training available. Sign up, take some "target parts" with you and sample their guns and let them advise you on the best way to use their stuff. It's in their best interest to make you a good painter.... ALSO they are so used to selling to "car body guys" that many of them look forward to helping "unique users" for a change of pace in their life.... You 'll probably get handled very well by an old timer, so look for one of them........

If any of you have experience doing a complete catalyzed urethane ( and I've often seen the name lacquer used with urethane so the word lacquer really does not mean specifically very much )I'd be interested in seeing a general scenario of the spray process compared to nitro, which is all I've ever used. Thanks, Bob

Bob,

I've used PPG poly, Dupont 2-part and HOK acrylic enamel (which is a multipart finish - in addition to their 2 part primers), and have also help spray PPG lacquer. Yes, automotive, but entirely different.

The catalyzed finishes are WAY more technical, and leaves little room for material adjustment. As you know, lacquer can be thinned all you want, and viscosity adjusted along with gun adjustment. With cats, you gotta have a good gun, and be able to adjust it. I would say lacquer is to cooking what cats are to baking - with baking, you usually need a specific recipe for a good final product, where cooking you can adjust along the way.

Did some test shots of McFadden's urethane, and its what I expected - much less forgiving than lacquer, much faster drying, and just, different. Not really something I'd want to do if I wasn't adept at spraying.

As for the process - a friend taught me his recipe (includes a sealer for color), which basically consists of West System to fill, and straight to finish. Less coats, harder sanding - which is why its good to be adept at spraying straight off the gun, or you'll be leveling forever. McFadden's tech will tell you either seal then topcoat, or direct top coat. I personally like mechanical linkage, hence the West fill.

One issue I have with cats is that if you sand through, you're stuck with witness lines, then you start over. But, because it dries in a couple of days, theoretically, you could redo it and be done before a lacquer finish. So, I just gotta make sure I fill properly.

One caveat - smells almost as bad as what you've been going through.

Thank you all for the great help! I think I will be alright with a smaller 0.8 tip and a good gun. I was considering a cheap import type, but now consider something along the lines of a (very expensive) Sata minijet. As I can get different size caps, and it has all the adjustments.

Better save up for it fast..

Aaron,

The particular CLOU finish I am talking about is a clear catalyzed acrylic-urethane formulated for use on wood.

Eric,

The one thing you need to watch with the west systems fill or with any epoxy is the buffer temperature. If you let it warm up at all it will expand slightly and then sink back into the grain as it cools. You'll think it's shrinkage in the urethane but it isn't, it's the epoxy in the pores pushing the urethane up and then pulling it back in after you sliced off too much.

Oh -one last thing is to not sand too fine, stop at 220, both before the epoxy and before the urethane. Wash down the sanded surface with thinner and shoot. After your topcoat, if all goes well, you can actually level with 1500 and go straight to the buffer (but let it gas off for a week or two first after leveling in case there's shrinkage). You pay more for the high solids but it's worth every penny in time saved later on.

Thanks for the advice David. Do I really need to fill with the epoxy if I shoot closed grain woods like alder? The first project that wil be done (after some practice) is an alder body with a bloodwood (satine) top + a maple neck with bloodwood fingerboard.

Is the epoxy merely for filling grain, or is it necessary for proper adhesion?

I want to obtain the thinnest possible film thickness that will still give a high gloss, like on prs guitars.

Eric,