What is the best polish compound to use?

There are many good brands out there. In the UK, professionals tend to use 3MFarecla and Autosmart Evo, but there are very many others out there, such as Menzerna and Meguiars

Our best advice, if you are starting out, is to pick one brand and stick to it. This is because each range of products will combine into a system, from coarse cutting products to finishing products, complete with pads. You will then be working with a system which is tried and tested.

It is even better if you can go on a course, and use the products they suggest, because they will show you how to best work with that system, which will include the pads and other ancillary products. 

When you have become competent with that system, you can try other products and begin to mix-and-match.