Is topcoat one word?

Yes, topcoat can be one word, but it can also be written as two words top coat, depending on context and style guide, and both are accepted. As one word topcoat, it is commonly used in painting, woodworking, automotive, and nail care to mean final finishing coat, and in fashion to mean long overcoat worn over suit, for example, topcoat paint, topcoat clear coat, topcoat for nails, and 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. Dictionaries list topcoat as one word noun meaning outer coat and also meaning final coat of paint, and top coat as two words is also understood. For example, Merriam Webster lists topcoat as one word. So you can write it as one word topcoat when referring to product or garment, and as two words top coat when describing layer, but one word is more common in industry terminology, especially in painting and fashion. So yes, topcoat is one word and is correct spelling, though top coat as two words is also acceptable and means same thing, with one word being more standard in technical use.