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Topic: Clear Coat |
Rick Collins
From:
Claremont , CA USA
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Posted 28 Jun 2003 7:42 pm |
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I was unsure of where to place this post:
For the experts in finishing:
When clear-coating over a color finish, is polyurethane or nitro-cellulose more durable?
Many Thanks, Rick
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Del Rangel
From:
Clayton, NC
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Posted 28 Jun 2003 7:52 pm
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| This is one of those questions were a number of points of view exist--kind of like which color is best. In my estimation I go with nitro for overall appearance and durability. To me nothing looks as good. Also, nitro can be touched-up and shot over with good results to cover repairs. You can most often melt an old coat and newer coat together. I don't think you can really do that with a poly finish. Just my .02 anyway. |
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Mike D
From:
Phx, Az
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Posted 28 Jun 2003 8:14 pm
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The main thing is to use compatible finishes.
Otherwise I'm with Del, Nitro is easily repairable. Poly is technically more durable but you're stuck with whatever happens to it. Superglue can be used to fill dings and such, but it's not an invisible fix. |
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Denny Turner
From:
Oahu, Hawaii USA
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Posted 29 Jun 2003 12:59 am
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Rick,
I've been spraying guitars for 38 years. Several months ago I saw a bass I hand-carved and sprayed with white Imron polyurethane (yee ghads) 27 years ago ... still holding up great.
It is near impossible to answer your question without knowing what kind of paint your color coat is and how thick and how old it is.
But if you're a reasonably experienced painter, then in general I would agree that Nitro is better overall than urethane.
Not knowing your painting experience level, but while I'm here, and in an humble effort to help, although I might be saying things you already know well: A good clear coat depends ALLOT on what your present color coat is. The color coat can require different kinds of prep for both what it is and what some clear coats (even different brands / formulas) can require for a good job. Knowing more about your color coat can help know if the clear coat will be a chemical or mechanical bond, both of which require different prep work and proceedure. Some top clear coats might actually chemically fry (wrinkle, orange-peel, destroy, etc) the present color coat if they're not compatible. Age and thickness (amount of being cured) of the color coat has allot to do with whether the color and clear coat you decide you want can be made to be compatible by technique. There are many other considerations in clear coating over and existing coat. In general, I prefer Nitro if I'm set up for it. Polyurethane has some serious consequences from errors. However, I have also had very good results with even urethane aerosol cans, but I know what I'm doing. If it's an important instrument / job, then it is commensurately important to read up on the proceedures in a good book and do it right. If it's not an important instrument / job, then an aerosol can of high quality CLEAR (not tinted) Polyurethane spray can give pretty good results IF DONE CAREFULLY AND PROPERLY. Learning the "tricks" is the only way you can do it in any case without chancing mistakes that can require sanding it all back down or stripping it (which can stain the wood if you're counting on see-through grain) and starting over. And in the case of polyurethane, you've just about got to cure it before you can sand it back down because it balls up on the paper when not cured, requiring stripping if you need it anytime soon. Etc etc etc.
Stewart McDonald's Shop Series book "Guitar Finishing Step by Step" is a good book that covers ALLOT of finishing technique specific to guitars. $24.
Please feel most free to contact me with any questions you might have.
Aloha,
DT~
[This message was edited by Denny Turner on 29 June 2003 at 02:24 AM.] |
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Erv Niehaus
From:
Litchfield, MN, USA
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Posted 29 Jun 2003 6:12 am
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I like to work with the nitro-cellulose lacquer. I am refinishing a Fender Stringmaster using the products from Guitar ReRanch in Texas. If you are interested, their web site is www.guitarrefinishing.com
They have duplicated all the popular colors and sell them in aersol cans. Very east to work with.
Erv[This message was edited by Erv Niehaus on 29 June 2003 at 07:14 AM.] |
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Dr. Hugh Jeffreys
From:
Southaven, MS, USA
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Posted 29 Jun 2003 6:27 am
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| Rick - I speak from the stand-point of boat maintenance.I've owned every type of boat except ferro-cement. Modern boats are clear coated with a nylon clear coat. The other products spoken of are fine, but for a really lasting coat, and since weather is not a serious factor, bar-top epoxy is great; so is top grade polyester resin. Be careful of some of the other products. They will turn yellow which is really noticable over a white finish. HJ |
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Andy Zynda
From:
Wisconsin
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Posted 30 Jun 2003 5:24 am
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I found this out about finishes for guitars:
Nitro is very thin, and very light and tough.
Poly is very tough, but thick & very heavy.
Heavy finishes will "deaden" the object it covers, reducing it's ability to resonate.
For this reason, acoustic and many electric guitars do not get along well with poly's unless applied VERY thin, thereby losing some of it's ability to protect.
Nitro stays very thin and light, even with multiple coats, so it is usually the finish of choice.
I airbrush electric guitars, and have always used poly, but when my detached garage is built and I have a proper spray booth, I'll be switching to Laquer.
(pre-existing finish compatibility set aside, anyway. Obviously, if you're overcoating an existing material, you gotta use something compatible or wind up with a disaster.) |
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Erv Niehaus
From:
Litchfield, MN, USA
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Posted 30 Jun 2003 5:37 am
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Del sent me the correct web site address.
It's www.reranch.com
Erv |
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Denny Turner
From:
Oahu, Hawaii USA
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Posted 30 Jun 2003 9:18 pm
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To test an undercoat compatibility with an overcoat, ... if the pickup cavity was painted in the previous coat(s), then tape of everything but the pickup cavity, and spray into the cavity to see if your overcoat is going to fry the previous coat. If it does fry it, no biggie; Just let it dry and the frying will usually be a stable coat of boo-boo but little consequence, although sometimes it crumbles the coat being overcoated and some "repair" might be indicated. You can also reasonably reliably test a coat's resistance to an overcoat product by wetting a rag with it's thinner / reducer and test the pickup cavity where any damage from probing with the wet thinner / reducer would not be noticeable. BUT ALWAYS TAPE OFF THE INSTRUMENT, because even fumes can damage a coating, and Murphy IS looking over your shoulder for touching or dropping-on the wrong place at the very worst time!
Unless the previous coat is polyester or acetone compatible, I can almost guarantee that an acetone base like polyester resin is going to damage the previous coat unless the painter is an expert or very experienced in doing that operation. Acetone is not quite as "hot" to other incapatible coatings as laquer is, but it's darn close.
Laquers over enamels will almost always fry the enamel, unless the enamel is modern-tech automotive paint specifically forumulated to be resistant to laquer.
One part enamel will overcoat just about any undercoat ... as will many 2 part enamels; but care must be taken to MAKE SURE, by asking the vendor, AND by reading every word on the label AND by testing the pickup cavity. Murphy lives for incompatible paint coatings!
However, by applying EXTREMELY fine mists of an incompatible overcoat and allowing EACH ONE to fully dry, the initial and dangerous layers of overcoat will only react to a minute depth of the coat being overcoated (not enough to fry it), and can slowly be built up to where the chemical depth-reach is only happening with the previous coats of overcoating. Full drying between each mist is critical because a build up of suspended vehicle (thinner) can "fume" down into the undercoat if it's not allowed to fume out with each coat. But this technique of applying incompatible coats really requires knowing what you're doing; which is not extremely difficult by thoroughly studying good literature specific to what you're doing.
In such a case with polyester over incompatible undercoats, polyester resin MUST be thinned with styrene, and NOT acetone which is the hot frying monster to most other paint products (although acetone will remain in polyester products' suspension until dried). 2 part paints also usually have a reducer of a different formula than their thinners and should be similarly respected.
One of, if not the most, important aspects of applying a coating you know how to apply, is in preparing the surface to be coated. Again there is no substitution for good literature specific to the operation, AND the vendor's advice, AND the labels on the overcoat product. Most vendors have additional literature about their products with more info than the label, if you ask for it.
Aloha,
DT~
Guitar Bedlam
[This message was edited by Denny Turner on 30 June 2003 at 11:10 PM.] |
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