Sweating Varnish

¶ Sweating Varnish. Gloss varnish which has been rubbed to a dull finish will sometimes change back to a greasy gloss and that is called sweating. The most common cause of this defect is the application of varnish over under coats which are not sufficiently dry and hard. Sweating may occur over varnish under coats or over color and paint coats which are not dry. In some cases varnish will sweat, even when applied to properly dry and hard under coats, if it is rubbed too soon and before it is hard and dry. The long-oil varnishes, those which contain a large proportion of oil, are much more likely to sweat than the short-oil, hard-drying varnishes. The long-oil varnishes are, of course, the most durable type but are not intended for much rubbing. When they are rubbed they should be bone dry and hard. The very best quality finishing varnish is likely to sweat if rubbed as soon as a week or two after application. A much longer time must be allowed before rubbing such varnish with safety.

¶ When it becomes necessary to rub finishing varnish of the long-oil type the job should be cleaned up after rubbing and be allowed to stand five or six hours in Summer or over night in Winter, before another coat of varnish is applied. Usually if sweating is going to occur it will show up in that time. And when sweating does occur allow the job to stand just as long as possible before 'brushing on the next coat. Also rub it again lightly with very fine pumice or an old pumice rag before application of the next coat of varnish. When a job is so handled that sweating may occur it is well to use another precaution, that of applying the rubbing coats as carefully as finishing coats are put on, as to brushing, and rub as lightly as possible, using plenty of water. In this way the under coats are not cut deeply by the rubbing and a deeper lustre will finish on the job. If this procedure is not followed or if the job is finished too soon the finished surface may be specky or full of brush marks.

¶ Green varnish, that which has not aged enough, is often the cause of sweating, but when using high quality standard brands of varnish this is a very remote possibility and the cause of sweating is more likely to be one of those suggested above or rubbing too long and too hard in one place which heats up the varnish even though it was hard and dry enough to begin with.

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