For the cavities like inside the door and seals you need waxoyl spray and you need to warm it up before using it. Best thing is to thin it down using white spirit. You wont need much The the underbody you need to clean of the dirt. Using a good brush and elbow grease. I used a paint on waxoyl underseal. I bought a 2.5 litre tin and its way too much. 2 coats are good enough. The underbody and arches and pretty good when it comes to corrosion on starlet, make sure you arch edges get good clean and coating. The rear beam, petrol tank and wishbones rust the most in my opinion, so work hard on them. Ideally removing them and cleaning them up and then putting back in works well. I used 2 coats of hammerite direct to rust paint in black and put 2 coats waxoyl coating on top. Checkout my thread if you want.