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Finish Compatibility -- from our friends at NOFMA:
Q: What could cause polyurethane to blister and peel into spots?
A: Several things can cause polyurethane to peel in limited sections of a floor. One of the most common is surface contamination. Polyurethane will not adhere to wax, lacquer, shellac, oil soap residue or products which contain sterates. Other tradesmen could have caused the problem if they dropped a contaminant that went undetected before the polyurethane application.
Additionally, if stain is not sufficiently wiped off, leaving a heavy film on the surface, the finish could blister. Failure of a previous coat of polyurethane to dry or skipping the screening process between coats can also cause blisters. If the blisters are on the surface only, you should be able to screen and recoat. Delaminating from the wood's surface will require complete sanding and refinishing.
Q: What types of floor finishes are compatible with each other?
A: As long as the existing finish coat is completely cured, the topcoat finishes commonly used in the hardwood flooring industry, e.g. oil-based, water based, moisture cure, acid cure are generally compatible with each other.
The finish is considered "completely cured" when the film forming reactions (cross linking) are substantially complete. This is different from the "dry time" or "recoat-time" which is typically stated on a product label. Despite the fact that most finishes are able to bear traffic in a short period of time, the curing phase of a finish (both oil and water based) can take several weeks.
Q: How long should I wait to ensure that the finish is completely cured before recoating with a different system?
A: If you are planning to intermix finish systems, industry guidelines recommend that you wait at least 30 days before coating with a different finish system. Proper intercoat surface preparation becomes even more important in these situations. Make sure to follow the manufacturers' recommendations concerning surface preparation before recoating. It is a good idea to check compatibility by first testing a sample area. Remember that coatings such as lacquers are traditionally formulated with resins which behave differently from urethane resins, expanding and contracting in response to temperature changes at different rates. The combination of these two dissimilar coatings with floor traffic can ultimately lead to bonding problems with the finish.
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