Mixing finishes

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Author Message Steve Taylor Senior Member Username: steveatwi Post Number: 34 Registered: 07-2008 Posted on Friday, March 18, 2011 - 06:18 pm:    I have recently seen a job where they used a UV finish on veneered panels, then refinished them with lacquer on the job. There have been problems and they would like to know the cause. One party suggests that this combination is bound to fail. Another says they do it all the time. Help! Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 932 Registered: 03-2003 Posted on Tuesday, March 22, 2011 - 10:55 am:    Refinished? Do you mean the first finish was stripped and then a new finish applied, or was another coat applied over the first coat? Was the UV finish factory applied? Either way, if the new coat did not have the UV inhibitors, then it had no protection, thus, it would fail if exposure was severe enough. Ron Geren, AIA, CSI, CCS, CCCA, SCIP www.specsandcodes.com Bradley Roa, Director of Architectural Services Junior Member Username: bradley_roa Post Number: 2 Registered: 08-2010 Posted on Tuesday, March 22, 2011 - 11:21 am:    I will jump in here. Steve and myself are inspecting this job together, so I can answer your questions and add a some infomation. The UV coating was factory applied to flat panels. The finish was not stripped, but reportedly scuff sanded, and then a lacquer was sprayed over the UV finish in the field. The work is indoors in a low to extremely low humidity environment. Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 933 Registered: 03-2003 Posted on Tuesday, March 22, 2011 - 11:36 am:    That clarifies some. But what is(are)the specific failure(s)? You can have UV radiation damage even in an indoor environment if the material is exposed to sunlight through a window, door, skylight, etc. There are even some man-made sources of UV radiation that could possibly affect a material. Ron Geren, AIA, CSI, CCS, CCCA, SCIP www.specsandcodes.com J. Peter Jordan (Unregistered Guest) Unregistered guest Posted on Tuesday, March 22, 2011 - 12:27 pm:    The term "lacquer" is used widely for any high gloss finish. When I was a lot younger, "lacquer" came from the lac beetle with an oil-based vehicle. More recently, there are acrylic lacquers (not really lacquer IMO). I think you really need to find out specifically what was used. I can see some grossly incompatible systems or some that simply aren't recommended with the real culprit being something else. Jim Sliff Senior Member Username: jim_sliff Post Number: 39 Registered: 08-2010 Posted on Wednesday, March 23, 2011 - 04:23 pm:    Almost every lacquer made since the 1950's contains acrylic resin in addition to (or in place of) nitrocellulose. Pure nitrocellulose finishes based on urushiol are extremely difficult to manufacture - also highly explosive during the process. The addition of acrylic resins provides longer-lasting finishes with the same low mil thickness and rewetting properties of urushiol/lac beetle/Asian/nitrocellulose (pick your poison - all fall under the "lacquer" generic label) lacquers; all of them, "acrylic" or not, melt themselves into the previous "coat" - an important quality. This "melting" process is controlled by the method of application, normally conventional or HVLP spray. I do know of one manufacturer that makes a line of aerosol-can lacquers that spray as well as any conventional-spray lacquer (applied via pressure pot or cup gun). They designed a special tip and the results are truly remarkable. I finish musical instruments as a hobby and have not used my conventional equipment in several years. Anyway - each "coat" of lacquer consists of (usually) 2-4 "passes" of the spray system over the surface. Each pass is VERY light, and each "coat" may only be .5 - 1.0 mils wet (and .5 or so mils dry, depending on solids-by-volume). To get a really fine, rich finish takes 4+ coats (I like at least 6) with sanding between coats after the second or third using progressively-fine wet-sanding techniques (800-2000 grit wet-or-dry paper) and a final polish/buffing, with or without a buffing compound (NOT rubbing compound, which will remove all that nice lacquer!) OK, sorry - didn't mean to sidetrack into "lacquer 101". Without seeing pictures or knowing what the "failure" is, I'd assume there's an intercoat adhesion issue between the UV-cured material and the field-applied lacquer. In other words, it's peeling like a snake. The cause would be insufficient preparation - that the UV-cure coating was not properly prepared before the lacquer was applied. UV-cure coatings can be can be extremely hard finishes or soft, but regardless lacquer will have no chemical bond with it (as it would with a lacquer sanding sealer). The lacquer also also not reach the wood itself, so it can't penetrate the grain for a mechanical bond; and since the UV system isn't a lacquer the lacquer won't melt into it. The only adhesion qualities possible will be mechanical - and that means more than scuff-sanding. "Scuff sanding" is one of those terms that is commonly used but rarely performed adequately in the field - there is NO firmly established standard for scuff-sanding. Technically, if the applicator takes any grade of sandpaper and rubs it along the surface creating some scratches that mar the finish he's met the Specification for "scuff sanding" - unless one or the other of the manufacturers describes the process differently *in detail* or it's clearly specified. Performed according to the example above (random marring), though - it'll fail. I would have specified sanding the surface to a roughness equal to or rougher than that of 220-320 grit sandpaper. That gives the applicator something to base the required work on, provides a rough enough surface for mechanical adhesion of lacquers (assuming no contamination of the surface AND light-duty conditions). It also gives an inspector something to work with. There are ASTM test methods that could be adapted but there's a 99% chance they'd never be performed. Rarely does a coatings contractor possess copies of ASTM documents, nor do they have the required test equipment. Adapting standards is usually a bad practice. The kind of requirement I mentioned, while sounding like it's "shooting from the hip" can actually be performed and verified - but, like any other requirement, if you Spec-and-run (with no field inspection requirements) it really doesn't matter what you specify. Unfortunately there's no quick-fix. If the lacquer is peeling it'll have to be completely removed (which may not be too hard), the surface prepared properly and recoated. Usually the applicator is blamed - but with no established standard and a specification using general terms it's really a specification issue. However - the applicator SHOULD know better UNLESS he was not aware the shop-applied sealer was a UV-cure system and not a lacquer. If the finish is to be a lacquer the logical sealer would be a lacquer - so IMO the Spec should have noted the UV-cure material essentially as a substrate in the paint Section; the painting contractor often never sees the fabrication Sections and may have had no idea a non-lacquer was used. Steve Taylor Senior Member Username: steveatwi Post Number: 35 Registered: 07-2008 Posted on Wednesday, March 30, 2011 - 02:39 am:    Jim, Thank you for your excellent information. The lacquer was applied over the acrylic finish for two reasons: there were problems with the original finish because the job site was not closed up or climate controlled when the woodwork was installed, and the design team changed during the project - the new decorator wanted a higher gloss. I thought that, as you said, there would be no chemical bond between the two finishes, but that a mechanical bond could be adequate if the surface was properly sanded. My belief is that the current problems are primarily the result of the building not being closed up and climate controlled during the installation of the woodwork and while the refinishing was done. In some areas inadequate preparation may have contributed. Jim Sliff Senior Member Username: jim_sliff Post Number: 44 Registered: 08-2010 Posted on Saturday, April 02, 2011 - 06:13 pm:    Yeah, sounds like a lose-lose situation. The new decorator, IMO, could have looked for a compatible gloss acrylic finish that would have been more "forgiving" in a recoat situation; there are plenty available. And, as noted, preparation was most likely a contributing factor - lacquer just doesn't get much "bite" and usually works best only over itself (and why lacquer-type sanding sealers are a good choice for new wood if lacquer is the finish...due to the "melting" properties intercoat adhesion issues don't exist.). My guess based on the design team change is that there was a disconnect - either the right info never got to the new decorator or...well, sometimes assumptions of compatibility are made in the wrong situation.