Is it topcoat or top coat?

Both topcoat as one word and top coat as two words are correct and widely used, and both are accepted, though usage varies by industry and style guide. As one word topcoat, it is commonly used in painting, woodworking, automotive, and nail care industries to mean final finishing coat, and in fashion to mean long overcoat, and is listed as one word in dictionaries like Merriam Webster. For example, topcoat paint, topcoat clear coat, wool topcoat. As two words top coat, it is also correct and often used in British English and general descriptions, for example top coat of paint, top coat of varnish, top coat of nail polish. Many people write it as two words when describing layer, as in apply a top coat. In technical product labels, one word topcoat is more common, such as polyurethane topcoat, clear topcoat. So you can write it either way, topcoat as one word is more standard in professional and product terminology, top coat as two words is more descriptive and also acceptable, meaning same thing, final finishing layer. So answer is both are correct, one word is more industry standard, two words is also understood and used.