Drying times vary strictly because of the chemical formulation, but watch out for very special drying instructions about when you can apply a second coat. With some varnishes, you can apply a sccond coat after 4 to 6 hours of drying. If you do not get around to putting on the second coat in those 2 hours, then you must wait 24 hours before applying the next coal, This might sound odd, but it happens because the second coat produces a bonding action 4 to 6 hours after the application of the first coat — which is good. After 6 hours, however, the first coat becomes too hard to permit the bonding action, so you must wait until drying is complete to apply the second coat. And then you must do a light scuff sanding before putting on the second coat, to create L'tooth" for the new coating to grab.
Unless special drying instructions are indicated, varnish should be allowed to dry thoroughly between coats. The instructions on the can tell exactly how long. Usually, 24 hours between coats is a safe time, but this can depend on the humidity in your work area. If the air is humid, you should leave the work set for 36 hours to ensure a safe drying time.
The drying of modern varnishes is a two-step procedure. They dry and harden in the first step: they cure in the second step. During the latter part of the drying period, the varnishes look dry and feel dry. That's when they are curing. If you apply a new coat too soon, you slow down this curing process. As a general rule, allow the longest practical time for drying to get the best results. It doesn't hurt to extend the time to 48 hours or more.
Was this article helpful?
THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.