Clear coating application in wood furniture finishing

Clear coating is applied in the finishing process to form film layer to protect the color and surface underneath. The selection of the clear coating is mainly determined by the strength (protection) needed by the finishing layer. However the type of the clear coating used in finishing will also determine the look of the finish produced. So when we select the type of clear coating, we also need to consider about the final look of the finishing required.
In a certain finishing process the type clear coating used should be same from the first coat to the last coat. It is much more safe to use the same type of clear coating materials from the wash coat as the first clear coat, sealer and the top coat as the last coat. It is better when we use the same supplier for all finishing material used including the clear coatings, stain and thinner for a certain finish. By using the finishing materials from 1 supplier then we can ask for guarantee for the whole finishing system to the paint supplier. If we have to mix the finishing materials form many supplier, then we really need to understand about the characteristic of each finishing material.

There are many types of clear coating based on his main resin, that each has his advantages and disadvantages properties. The types of coating that is commonly used in wood furniture finishing are:

- Nitrocellulose (NC)

- Acid curing or melamine

- Polyurethane (PU)

- Ultra violet cured coating (UV Coating)

- Water based coating

The use of clear coating with different types form the bottom layer to the top layer actually is not recommended. The different character of the clear coatings will make the process more difficult. The finishing process must be done more carefully and accurately. The uses of many different type of clear coating may cause problems in the finishing film layer such as: crack, frying, adhesion, etc. The incompatible clear coating will also cause problems in the finishing looked such as: clarity, lustrous and yellowing. 
When we decide to use different clear coating in every layer of a finishing system, we must consider the material compatibility in the whole system. One simple rule, is : never layering the materials that are dried with chemical reaction on the coating materials that dry by thinner evaporation. The finishing materials that dry by reaction are: polyurethane, UV coating, acid curing and precatalyzed coating. While the coating that dry caused by the evaporation if his thinner without any chemical reaction are: nitrocellulose coating, butyrate coatings, vinyl coatings, water based coatings. The finishing traditional such as shellac, of varnish is cured by evaporation of his thinner. 
The second rule is the application on the chemical reaction dry film should be done when the coating layer is already fully dry. 

Here is the table of compatibility of various type of clear coating materials in application.

Material	Compatibility	Remarks
NC on AC	NC on AC	AC must completely cured
NC on PU	OK
NC on UV	OK	UV must fully cured
NC on Water-based	OK	Water-based must completely dry
AC on NC	Not OK
AC on PU	Not OK
AC on UV	OK	UV must fully cured
AC on Water-based	OK	Water-based must completely dry
PU on NC	Not OK
PU on AC	Not Ok
PU on UV	Ok	UV must fully cured
PU on water-based	Ok	Water-based must  completely dry
UV on NC	Not Ok
UV on AC	Ok	AC must completely cured
UV on PU	Ok	PU must completely cured