So I did wanted to start with the electrical problems (mainly the cables are not installed) but the instructions of the manual and workshop book are not precise enough. I will come back to that later when I can find some more information.
Any way, I bought in internet some paint and undercoat for the front and rear fender (they have some scratches but that will have it own chapter). Before trying it with a 300€ part like the fenders I decided to give a new life to the rusty and scratched air pump.
Mainly you will have to follow always the same steps for any kind of paint on any metal surface:
1. Protect the surfaces around the part you are going to paint, you don't want to ruin any oder surface by trying to fix the bad surface. Painters tape are useful for this if you the surface is not big. If it is big you can cover it with paper or plastic and seal the unions with painters tape.
2. Using sandpaper the paint will come off easily. Make sure that you are using one with low number at the beginning (180 will be fine) and one with a very high number at the end (600) so that you end with a perfectly smooth surface with no traces of the sandpaper. Don't rush, the more homogeneous you do your job, the better surface you will have in the next step. Make sure that any rust of scratches are out.
3. Clean the surface. Wait a bit until all the dust in the air is on the floor and clean the surface completely with a clean cloth. Use some kind of alcohol and make sure that you clean all the surface. Clean it slow, with a move pattern and if posible from up to down (so the dust will always go down).
4. Take precautions, put on your mask and glasses for the next step. Make sure that you are in a well ventilated area and cover your hand with gloves (of you think you may need extra protection clothing feel free). Protect the space where you are going to paint, you don't want a new color on your wall or floor.
5. Undercoat or primer coat are very important to protect the final color and the surface. It will last longer and the rust won't appear so easily. Using an oscillating hand move and never stopping on a specific area paint the surface lightly. Better to give many layers than having drops of paint going down the surface. Let it dry as market of the spray.
I personally prefer the spray rather than the brush as the final result is much smoother and level.
6. Paint it with the final paint. In the BMW R25/3 case the final paint is RAL 9005. Following the same steps as in the before step you can paint your surface with (if possible) more than one layer. Don't get crazy, cars are painted with one or two layers. No need to have 100 layer painted surface.
Once finished leave it drying for enough time. You don't want to loose that fantastic shiny new paint for touching it too early. So far so good, we will continue working.