I own a horsebox which has been painted with Hammerite smooth black paint. I wish to repaint it to a sage green colour with a cream top. I am planning to sand the current surface but am unlikely to be able to take it back to bare metal as I am not a professional and will be doing the work on my drive, exposed to the weather. What type of primer should I use once it is sanded or can it just be painted onto with several topcoats. The job will be hand painted with a roller and hand brush. Are any of your products suitable for this type of application and if so, what do you recommend?