The degree of success you will achieve with any paint job depends to a large extent on the preparation of the surface prior to the actual painting. Future problems can be minimized if care is given to properly prepare the surface.
The first step before even applying the primer is to make sure you have a totally dust and grease free surface. If there are spots that you suspect may have any oily substance on it, the area can be cleaned with mineral spirits. Be sure the area is dry before proceeding to the next step. If you are using an oil (alkyd) based paint, wipe down the surface with a tack cloth, which is a commercially prepared cheese cloth that is sticky and thus attracts the dust to its surface. If you are going to use an acrylic based paint, wipe the surface down with a barely damp lint free cloth.
The primer you select must be compatible with the surface it is to be applied to. It must also be compatible with the type of paint which is going to be used to decorate the animal. The adage is "oil and water do not mix". Read your labels, if it is oil based it will clean up with a thinner or mineral spirits. If it is water based it will clean up with soap and water.
Early carousel manufacturers used oil based paints laced with lead for coverage and opacity. It was mixed to a thick, creamy consistency, applied, and was thick enough to eliminated the task of sanding the carving. Unfortunately, it also filled in the carving details. The carvers knew this would be the result so they carved "deep" to offset the effect of the primer. Allen Herschell used a thin milk-paint which penetrated deep into the wood fibers before priming. This milk-paint is impossible to remove as it has penetrated deep into the wood fibers. Some even pre-coated with shellac. The purpose was to seal the wood before applying the primer coat.
This "paint stew" was then applied liberally to the newly carved animal resulting in a warm red under color for their golden- hued animals. It provided a flat or matte finish (for tooth)with great hiding properties and even U. V. (ultraviolet) protection due to all the pigment present.
Make sure the primer you select is also compatible with the surface on which it is to be applied. A wooden horse is compatible to either a water-based (acrylic) primer or an oil (alkyd) primer because it is a porous fiber. The oil based primer is less likely to raise the grain of the wood. An aluminum horse must, however, be primed with a special metal primer which usually contains aluminum oxide. A plastic, fiberglass, or resin based horse must use a primer that is formulated to adhere or bite into the surface and yet is compatible with the paint to be applied on top of it. Gesso is sometimes suggested as a primer for a plastic, fiberglass, or resin figure. Although gesso is an excellent primer for any absorbent type of surface, such as canvas, paper, or even wood, it will not adhere properly to a smooth surface such as plastic. It does not penetrate into the pores of the surface nor expand or contract at the same speed as the surface and the succeeding layers of paint. The result is that the paint will crack and pop off. Even if you pre-sand the surface to attain tooth, you can not attain a good bond. By sticking to the same paint type, each layer of paint will bond with the previous layer. Each layer will also expand and contract, or move at the same speed.
If you have doubts on what primer to use, read the label and talk to your paint dealer. He often has more information than is printed on the label. The key is adhesion and when dry the paint has a matte or a flat, porous surface. Apply your first coat thinly for penetration, sand to eliminate the furring created on the cross grain cuts and then apply second or third coats as needed. The furring will decrease as you fill the end fibers with primer. You want to achieve a total even color and a total smooth surface. If possible, use a spray application which can utilize automotive type primer or quick-drying shellac based primers. You can also tint your primer, dark for a dark horse, gray, tan, and so on.
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