not sure about the inside, but on the underside of my floor I had some synthetic enamel mixed at a car paint factors, I then applied this with a foam roller I got from the pound shop, finish was very good, I am not a fan of hammerite products, as years ago I painted a mini underside with it and discovered it covered well, but it did not adhere as well as I thought and rust continued under the paint although it looked fine, ended up scraping it all off and derusting, etch priming, stone chipping and top coating with normal synthetic enamel, had no issues with it at all since, and that was about 18 years ago.