July 20th, 2012, 07:27 AM
anyone ever done it? I had some Lacquor primer left over from another prject & did my wood prep & sprayed it & plan on using a Urethane base & catalyzd clear over it.....will it work? should i maybe spray a coat of urethane primer over the lacquor first? I had both primers but noticed last time the lacquor sprayed thinner & less hazy then the urethane so i decided to use that one instead....
Colt W. Knight
July 20th, 2012, 08:15 PM
There are no chemical incompatibillities in spraying urethane over lacquer, but to understand why you should or shouldn't is important.
Lacquer is an evaporative finish. It does not cure, the solvents simply evaporate. This is why you will hear people using the term "gassing off". The solvents evaporate from the outside in, so as the outer layer will harden quickly creating a hard shell if you will. This is why lacquer dries to the touch very quickly, but takes a very long time to actually dry because the outer layer retards the evaporation of the inner layers. You can witness examples of this by looking at older guitars finished in lacquer that look as if finish has shrunken into the grain giving it a wavy appearence. Because they do not cure chemically, subsequent coats melt together because the solvents in the fresh coat reliquify the previous coat creating one singular homogenous coat.
Urethane cures chemically ( a lot of them in 24 hours or less). Once it has cured, it is stable. Lacquer is also very stable when it has dried fully, but that can take months or years. Urethanes, once they cure, do not melt together into one coat. They actually stratify, creating layers. If you are finishing with urethane, its common to have a recoat window. Recoat window is the time before the urethane cures, so subsequent coats will adhere. If recoat windows are not met, it is typically advised to scuff up the surface to promote adhesion. By scuffing up the surface you are increasing the surface area the new coat has to adhere to. Some urethanes, especially oil based polyurethane ( like minwax) are notorious for leaving drastically stratified layers, and when you wet sand they can leave witness lines. Think of these as a topographical map. Each elevation of the layers is marked by the edge of the coat.
Another term you may hear is catalyzed and precatalyzed. This means the finish has a catalyst added to actually make the finish cure chemically or dramatically speed up drying time.
By spraying urethane over lacquer that hasn't fully dried, you are trapping in solvents. When the finish expands and contracts during temperature and humidity swings, the lacquer and the urethane will expand/contract at different rates causing checking, flaking, and delaminations.
July 20th, 2012, 08:41 PM