Filter default_content only for products

default_content is a filter used in the backend. You don’t necessarily have anything in the loop so standard functions will probably fail. However, you’re given a second argument of type WP_Post. You can check its post_type easily and work from there.

add_filter('default_content', 'WPSE_product_default_content', 10, 2);
function WPSE_product_default_content($post_content, $post) {
    if ($post->post_type !== 'product')
        return $post_content;

    $content="content to add to a post"
    return $content;
}