What is the difference between post-cat lacquer and conversion varnish? People and manufacturers seem to use the terms interchangeably sometimes and not others.
First thing to understand is that every manufacturer is simply a cook and a blender. The specific raw materials used to formulate the product will dictate how the product will perform not only in the container, but also during the application and curing process, and then in the long term (durability).
A post-cat lacquer typically requires the addition of a separate catalyst by someone other than the manufacturer. Once this catalyst gets added, the lacquer starts a shelf life of anywhere from four to twelve months, depending on its acid value content reacting with the other components in the formula. Within this timeframe, the lacquer will gradually increase in viscosity and may not atomize properly through your spray equipment.
This differs from a pre-cat lacquer, because the manufacturer incorporates the catalyst at the point of manufacturing, so it's built into the formula. A built-in catalyst can lead to potential problems if the manufacturer and/or distributor do not rotate stock properly, insuring fresh material.
A conversion varnish also requires the use of a separate catalyst added out in the field just as a post-cat lacquer would, but the main difference is in the time you as an end-user have to apply the product. Most CV products come with a stipulation of "use it or lose it." This refers to its pot-life, which is on average eight to twelve hours. Post cat lacquers offer a shelf life in months.
In my experience, post-cat lacquer has nothing close to a several month pot life. Pre-cat lacquers fall into this time frame, but post cats' typical pot lives are much shorter. The pot life after catalyzation of a post-cat lacquer typically does not extend past 30 days. With conversion varnish, it's as short as eight hours and as long as 48, depending on the manufacturer. Also, the resins in a good water white C-V (Rel-Plaz, Global Resistovar, MacLac CV-4000 series, Sherwin-Williams Water White Conversion Varnish) do not yellow over time. A very important consideration over pickled work.
I'm a big believer in post-cat products both from the standpoint of shelf life and performance. I don't find adding the catalyst a big deal (I use hypodermic syringes for this purpose) and the fact that the post-cast products have virtually an unlimited shelf life until catalyzed makes them very attractive to me.
I find little difference in spraying conversion varnish from pre-cat except that the conversion varnish, because of its higher solids content, requires me to use a smaller N/N set in my gun. In short, post-cat lacquer or C-V are, in my opinion, superior to pre-cat.