I have a 21ft aluminum boat i want to paint it the paint on it now is worn whatdo i need to do to paint this?

This is an easy one. Just go or call your local boat dealership. They usually carry products to improve your boat with many types of marine paint and might give you information. Many or all auto paints will also work. The preparation for painting is the same as painting a car.

A polyurethane will be the best option for your 21ft aluminum boat. It will last long time and it is affordable. http://www.charteryachtdubai.com/yacht-rent.html

A polyurethane or two pack system will give best results. The suggestion of Interlux or any other quality brand is good. These companies provide comprehensive guide sheets as to what coatings system to use, and the required preparation for each of these steps. What ever you choose, do the whole system, do not miss any steps. Unless you are set up to use these toxic coatings you should not consider spraying - the VOC and other payload can be fatal. To get a good result it will pay to start with a few smaller jobs to gain some experience with just how to get the result you are after. While painting as such is pretty straight forward, it takes many years to gain the skills and knowledge of a professional and gain that glass finish. The best advise I ever received when starting my painting career was that if you are doing less than 75% prep work, you are not doing enough. Today I rarely do less than 80%, and can spend upwards of 100 hours just prepping a small boat. The last commercial boat I did took 1200 hours of base preparation before the first top coat. Be sure you have the space and time to do a project of this magnitude, you will not get the result if you try and do it over a long weekend. However a good, fair job can always be done with planning and a clear idea of what it is you want to achieve. Part of that planning is doing the smaller 'test' jobs so you understand the requirements. Refurbishing a small tinny is a good toe in the water project, and allows you to make the inevitable errors and fumbles part of an enjoyable learning process - rather than creating frustration and upset. The skills you gain doing that will make the big jobs much less demanding, as you know what to expect. Car paints are pretty good, but you would not expect them to last when immersed - any more than a car would last wet all the time. Specific formulations for coating Alloy in a marine environment are sold through the good paint companies - just stick to the system and the one brand and you have a lot of backup and a company that will take a lot of effort to ensure you get it right - they want your future business. Stay safe. Mind you - I will never paint aluminium given the choice. The natural oxidisation protects better than any coating, and allows instant visual signs of early corrosion. That's the ex proffessional fisherman talking though I suppose.