One Flash, multiple exposure, need Help !!

Hi Everyone.
I got my wireless flash trigger yesterday, as the first part from my simple strobist system that I'm building (still waiting for light stands, umbrella and other Flash).
So for my first try ,I decide to go out and shoot a friend's car, using just one flash then mix the exposures into one photo in Photoshop.

First I started with 1/200 to have cool blue sky and I found that f8 go with it in my camera and EV -1.0 to get a pure black tone not gray for the car.
set the Flash to 1/2 power then to Full power because it was not enough.

the result of multiple photos was like this

• First problem I figure it was the unfixed background (Flag and boat are moving cause of the wind).
  

• second problem, I think it was better if I slow my shutter speed to get more of the sky and ambient light, not sure if it's the best solution but I found that the sky is a little dark.
  

• third problem, the aperture seems to be very small so the car didn't get enough light. well I mean to do it, cause I want much DOF and because the car is black I want just a small part and edges to be lit.
  

IMAGE LINK: http://www.flickr.com/​photos/3afsa/393446591​4/

IMAGE LINK: http://www.flickr.com/​photos/3afsa/393446497​4/

direct flash doesn't work well with black cars. You need to bounce the flash into something like a white board/sheet.

Thanks, I'll keep that in mind next time

ep88 wrote in post #12427895
direct flash doesn't work well with black cars. You need to bounce the flash into something like a white board/sheet.

Just so you understand, that doesn't mean a bounce card on the camera. It means something like a wall that will reflect into the paint on the car.
See the links in my Sig

oh !!! those links are very useful , Thanks

Did you ever think about adjusting ISO at all?

tristinGrind wrote in post #12435501
Did you ever think about adjusting ISO at all?

well, no, I use ISO 200 to minimize noise, specially because I use a low quality CP, so ISO200 with it is like ISO400 without, in term of noise.
But, what ISO can make difference when using it with Flash ?