Disintegration by reason of chemical action between pigment and binder is a matter for the paint-manufacturers' chemists to take care of; and this has already been done by all of the first-class companies with more or less success.

Abrasion of bridge paint, except by the before-mentioned cinder blast from steam locomotives, is something which should seldom occur, because nothing should be allowed to strike the metalwork of a bridge hard enough to disturb the paint thereon.

In respect to the action of deleterious gases, that question has already been discussed at some length herein.

How to Care for Incipient Failure of Paint

As all bridges should be submitted to careful inspection at short intervals of time, any incipient failure of paint should be quickly discovered. If the failure pertains to the field coats only, it will suffice to cover the defective parts with a layer or two of the final coating; but, if it extends into the shop coat, the spot affected should be covered with red-lead paint of best quality, and afterwards, for the sake of appearance, recovered with some of the finishing-coat paint. If the metal is exposed and rusted, which will not occur if the structure is properly examined at intervals of time not too great, some scraping will be necessary before the red-lead paint is applied.

By careful attention of this kind the life of the paint can readily be extended from twenty-five to fifty per cent, as compared with what it would be without such attention.

Determination of Time for Repainting

When the partial failures begin to come so rapidly that the retouching process is expensive and troublesome, and especially when the attacks appear to be liable to reach the metal through the shop coat, it is about time to give the structure a thorough cleaning and one or (better) two coats of paint. The psychological time for doing this can only be determined by an experienced bridge man—preferably the Superintendent of Structures of the railroad, state, county, or municipality.

Cleaning of Metalwork Preparatory to Applying New Field
Coats

The cleaning of the metalwork, if it has been properly cared for, will not prove to be a serious business; but, otherwise, it will involve a rather drastic operation. If possible, the shop coat of red lead should not be disturbed, but should be effectively re-covered.  In  places  the  torch may have to be used; but, as it cuts to the quick,  its  use  should  be  avoided  whenever practicable.  Similarly, the sand-blast  should  not  be  adopted,