Commas are used to separate adjectives that describe a noun equally. A comma takes the place of the coordinating conjunction and. Commas are not used with adjectives that do not describe a noun equally.
My old black watch is priceless. (No comma is used. Old describes age; black describes colour.)
His rusted, dented old car had served him well. (A comma is used to separate rusted and dented; both adjectives describe physical appearance.)
The sweating fat brown dog jumped over the fence.
(No comma is used. Fat describes size; brown describes colour; sweating describes condition.)
To see whether you need a comma between two adjectives, insert an and between the two adjectives; then read the sentence aloud. If the sentence doesn’t sound right, you don’t need a comma.