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BO40 and BO50 Brushable Urethanes

Eager Plastics offers a line of brush-on urethanes that have been used in architectural restoration projects of St. Mary of the Angels Church in Bucktown, the Chicago Cultural Center, restaurants and homes. The brush-on series has a convenient 1:1 mix ratio by volume and are thick enough so that they will not run on vertical surfaces. The shrinkage is minimal, maximizing the fine details that you will be able to obtain with your cast pieces.

BO40 is the softer version and BO50 is slightly stiffer. Both have superior abraison-resistance and have been used to cast plaster and cement, in addition to hard plastics. The materials are compatible and thus some people will first brush on a layer of BO40 for flexibilty and then back it up with the stiffer BO50. Both systems have a 20 minute pot life and cure overnight to an off-white color.

Porous originals such as wood, plaster, cement, concrete, stone, etc. must be sealed prior to application. Our sealing agent ER2310 works very well for this application. Also, because water causes curing problems for urethane, originals that may container moisture should be dried out before being sealed. Dense materials such as glass, pvc, steel, aluminum, acyrlic and sulphur-free clays do not need to be sealed.

Because urethanes are adhesive, all surfaces must have a release agent applied before brushing on the rubber. A simple and successful method is to use a liquid soap concentrate such as Palmolive. You should brush on 2-4 coats of soap, letting each application dry completely before applying the next. The surface should be slick like a bar of soap before brushing on the rubber. Some people have also used PVA as a release agent.

Applying the rubber is simple. After mixing in the correct ratio, a stiff brush should be used to apply the first (and very thin) layer. This thin layer is what will allow you to pick up the detail of the original. After 30-45 minutes, the surface will be tacky and you can use a spatula to apply layers of 3/8" - 1/4" thick. Four to six layers should be suffucient. DO NOT allow layers to fully cure before adding additional layers, as delamination may result. This system will not cure at low temperatures. A minimum of 65F must be present. In colder conditions, you may be able to aid curing through the use of a heat source.

24 hours later, you may peal the rubber coating off the original. For larger molds, you will probably want to build a mother mold our of plaster, fiberglass or Plasti-Paste. Brush on 40 and brush on 50 molds can be used for casting expandable foam products, urethanes, cement or plaster. For casting cement and plaster, try our release agent ER2320.

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