As Husafreak has said 800 will be fine with water, I only use 2k primer, which had to be mixed especially as I needed to use white to achieve the correct brightness of red on the 2010 Display.
Also if paint has hardner then it is 2k as it is 2 component, 1k is just one component i.e. the paint.
You don't want to 'plaster' the primer on, with the 2k primer I shoot one light coat followed by one wet and leave to dry. When sanding yes I take off as much as possible but you have to judge when to stop as you don't want to go through. If you look at some of the pics of the tail plane you can just see some of the original below the surface but I did not go through etc

When rubbing down the gelcoat I would have used around 600 grit to get a good key onto the fibreglass, there are special fibreglass primers that are flexible and provide a key for following primers, this type of primer is a very thin clear coat and I have used in the past when using 1k primer on glass etc but I have found the 2k primer which I use 700Ti if keyed it sticks like glue, its awesome and you have seen the finish with 2k over the top, preparation is EVERYTHING!

Also if you are using 2k or 1K for that matter use a air fed mask! I have heard of people waking up in hospital after using 2k with no mask!!! Not good for lungs!

Alan

Just for info I will be able, well after today (delivery today of a very nice AC/DC TIG welder) be offering a TIG welding service, this is a piece of equipment I have long wanted to own so I can weld Aluminium so will be able to repair and construct new projects etc
Many times borrowed TIG (Tungsten Inert Gas) units when on site on past but regs have sort of stopped this with contractors not allowed to use customers plant so decided to get my own..