Blistering of Varnish

¶ Blistering of Varnish. This defect is familiar to all, the swelling out of the varnish film in places after it is dry into bubbles or blisters. The cause of blistering is the action of heat, usually the sun, drawing out of the wood moisture or sap which has been sealed in by the varnish. Moisture in the under coats of paint, enamel, etc., and grease on a surface varnished are also fruitful causes of this trouble. Non-drying oil in the wood varnished will cause blistering. The action of the sun or heat from other source is that of heating1 and expanding the liquid under the varnish. The expanding necessarily tears the varnish loose to make room for the increasing volume of the liquid. The better the quality of the varnish the more elastic it is, and the more elastic the varnish the more likely it is to blister when subjected to these unfavorable conditions. Newly varnished surfaces should be protected from the concentrated rays of the sun or other heat source until hard and dry. There are on record cases where varnished surfaces have been blistered by the action of a bubble imperfection in a window glass. The bubble concentrated the sun's rays upon the varnish in the same manner as a burning or magnifying glass works.

¶ Shellac, especially, will not withstand heat. When used for under coats it may cause blistering when heated, even though no moisture is under it.

¶ Blistering is more troublesome, perhaps, on doors of residences, stores and public buildings which have south exposure. Some cases persist in spite of repeated refinishing. Stubborn cases are usually cured by stripping off all of the old finish down to the bare wood, using liquid paint and varnish remover or a blow torch, and then refinishing this way: For a stained and varnish finish, use a water stain, allowing at least six hours to dry and follow with from two to four coats of first-class exterior spar varnish. If a high polish is wanted, necessitating a well-filled, level surface, do not use filler but rather allow the first coat of varnish to dry two or three weeks and rub it down level with pumice stone and water. Clean up thoroughly. This will make the first varnish coat serve as a filler and it may be necessary to rub the second coat a little too, but be sure it is dry. If you rub long-oil, exterior spar too soon it will sweat and it will gum up and destroy itself. Be sure to use the same brand and kind of varnish for all coats, from the same can if possible.

¶ For refinishing such a job with paint or enamel, use the least possible oil from the start to the finish. The first coat may contain not more than ten to twenty-five per cent of oil and the balance turpentine. The under coats to follow should be thinned entirely with turpentine, making sure that each and every one is bone dry before applying the next. A first class enamel under coater may next be used or a coat of half lead and half zinc ground in oil and thinned with turpentine. The finishing coat should be first class white enamel of the kind that is made for exterior exposure, meaning a long-oil enamel must be used. Or, for the last coat mix with high class spar varnish a little of the lead and zinc under coater, just enough, to kill the brown color of the varnish, and use that as the enamel coat. If it is to be colored be sure to use colors ground in japan, not oil colors.

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