Poly over Lacquer?

Minwax site says no poly over lacquer. Do I have to sand to bare wood, or is there a better way?
Thanks, Jamie
Q. Can Polyshades® be applied over an existing finish?
Yes, as long as the previous finish is not lacquer or shellac. Be sure to test the selected color on a hidden spot to see how it will look over the original finish. To prepare the surface, make sure that the finish is clean and has had all wax and polish removed. Then lightly sand with #180 or #220 sandpaper. The surface is now ready to be coated with Polyshades®.
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Before I tell you what I did let me say in large capital letters: I'M NOT ADVOCATING THAT YOU DO THIS...BUT read on.
About 4 years ago I had several hundred feet of oak baseboard to stain and finish. Knowing even less than I know now (hard to imagine, I know!) I was told I could use Deft lacquer sanding sealer and Minwhacks poly. So I did. The baseboards look fine even though they get whacked by errant feet, the vacuum cleaner, and whatnot, for years. More than a year or two after I installed them I got out a magnifier to read the fine print on the cans and of course there in black and white, in type any ant could read, it says don't use varnish over the sealer. The Minwhacks poly says don't use over shellac or lacquer. So there you go, I couldn't have used more incompatible products if I tried. Did the baseboards self destruct? no. Did the finish flake off? no. Does it look 'bad'? NO! Would I tempt fate again, now that I know? NO WAY :)
So why do they insist you can't do it? Hell if I know, but I'll take them at their word and play their silly game. sigh.
dave
Jamie Jackson wrote:

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wrote:

Please allow me to boil down your response: ;-)
Are you advocating taking it down to the bare wood, or is there some sort of intermediate finish? I'll err on the side of stripping, unless someone tells me otherwise.
Thanks, Jamie
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I'd go down in major flames if I advocated that you put the poly over the lacquer. So regard it as an impossible marriage of products. Everything I've read says it's a no-no. I was merely recounting how I screwed up and did the "impossible". I won't do it again, but on the other hand, the base boards look fine.
Strip it to be sure, is what I'd suggest.
dave
Jamie Jackson wrote:

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On Sun, 02 Nov 2003 15:07:16 GMT, snipped-for-privacy@bigfoot.com (Jamie Jackson) wrote:

Some moisture cure polyurethanes are OK to use over lacquer (degloss first) but most solvent base polyurethanes can not be used over lacquer.
Call the industrial coatings tech support for manufacturers like Sherwin Williams, Target Coatings, etc. for the straight dope on this.
Don't bother calling one of the retail outlets.
Regards, Tom Thomas J. Watson-Cabinetmaker Gulph Mills, Pennsylvania http://users.snip.net/~tjwatson
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Another option may be to put a coat or two of shellac on top of the lacquer. I've put lacquer over shellac but not the other way around. I would think it would work. Then you could put the poly over the shellac. Others with more experience can confirm this would/wouldn't work. Cheers, cc
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In September of 2002 the New York City Department of Health and Mental Hygiene (DOHMH), Environmental & Occupational Disease Epidemiology Program (EODE) began receiving reports that Moisture Cure Urethanes (MCUs), which are used to coat wood floors in homes, were generating strong odors and raising health concerns for building residents. In response to these concerns, EODE reviewed material safety data sheets on various MCU products, researched the known health effects associated with chemical ingredients of these products, and consulted with the New York State Department of Health. Additionally, EODE, with assistance from the Office for Environmental Investigations (OEI), conducted an inspection during an MCU application at an apartment building in Brooklyn, New York on March 7, 2003, that included sampling for airborne ingredients of MCUs.
Exposure to the chemicals in MCU products can lead to a variety of health effects depending upon the level and duration of exposure. Brief exposures to elevated levels of these materials can result in headaches, respiratory irritation, allergic reaction and exacerbation of asthma; while very high and/or long-term exposures can lead to more serious health effects such as organ damage, reproductive effects, chemical allergies, and possibly cancer.
Environmental sampling at the Brooklyn apartment building detected several MCU chemical ingredients in the air. These chemicals produced noticeable odors throughout the building. The presence of the chemicals found in the common hallways of the building may result in irritation of the respiratory system, exacerbation of asthma in some individuals, and headaches. In an occupied residential building these conditions represent a nuisance.      Moisture cure urethanes contain a variety of solvents depending upon their formulation, and typically contain xylene, ethylbenzene and acetates. Many solvents, including xylene and ethylbenzene, have strong odors that can be smelled at very low levels. Short-term exposure to elevated levels can cause reversible irritation of the skin, eyes, nose, and throat; exacerbate asthma; and cause health effects such as headaches, nausea, and dizziness. In occupational settings and in animal studies, exposure to very high levels of solvents has been shown to cause neurological, kidney, and liver damage, and can impact developing fetuses. , The International Agency for Research on Cancer (IARC) lists ethylbenzene as a possible human carcinogen. IARC lists xylene as not classifiable as a human cancer agent.
Acetates, in general, evaporate more slowly and make up a smaller percentage of the product than the other solvents. There are many types of acetates. The acetate found in the air at the Brooklyn apartment building (ethyl 3-ethoxypropionate) is of low toxicity, but exposures to elevated levels can be irritating to the respiratory system. ,
Curing Agent MCUs contain isocyanates, typically toluene-diisocyanate (TDI) as a curing agent. This chemical helps create the hardness of the final urethane finish. During application and curing of the liquid MCU, most of the TDI is combined into the polyurethane without becoming airborne. Exposure to elevated levels of TDI in the air can irritate the eyes, nose, throat, and lungs and cause coughing, chest tightness, reactive airways disease, and shortness of breath. These symptoms can exacerbate asthma. Respiratory sensitization to TDI has been documented to occur in an occupational setting. Once sensitized, subsequent exposures can cause an allergic, asthma-like, response. Skin irritation and, less commonly, skin sensitization can also occur upon direct exposure to TDI. There is no information on the adverse reproductive or developmental effects of TDI in humans or animals (U.S. EPA 1994a). IARC lists TDI as a possible human cancer agent.
Urethane Polymers The TDI in MCUs reacts with a polyol (urethane polymer or co-polymer) to form the polyurethane finish. Urethane polymers are polyesters and polyethers. The chemical urethane (ethyl carbamate) is not a component of polyurethane products. Like the solvents, there are many kinds of polymers used. The specific polymers used are often trade secrets and may not be listed on product labels or material safety data sheets (MSDS). Urethane polymers are usually viscous and have a low volatility. Since residents do not come into direct contact with the product, exposure to these chemicals is unlikely.
Based on the findings of this investigation, which included review of scientific information on MCU ingredients and the inspection at the Brooklyn apartment building, DOHMH recommends the following:
It is recommended that the use of MCUs be avoided and an alternative product that contains less volatile and/or less hazardous ingredients be used. Safer alternatives should always be considered.

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